Introduce MutateAndPatchStatus helper for status writes

[jhrozek]

Summary

• The operator's status writes use r.Status().Update, which sends a full
  PUT of the status stanza and zeros any field a disjoint writer owns
  (e.g. a runtime reporter writing status on the same CR). Switch status writes from Update to Patch across all controllers #4633 tracks
  migrating to merge-patch; this PR introduces the helper the migration
  will call at every site so the DeepCopy-before-mutate discipline is
  captured in one place instead of re-implemented per call.
• MutateAndPatchStatus[T client.Object](ctx, c, obj, mutate) collapses
  the DeepCopy → mutate → Status().Patch(MergeFrom(original)) idiom to
  a single call. Plain merge patch, not optimistic-lock: status-subresource
  writers with disjoint fields must coexist without forcing a 409 on every
  overlap. Spec and metadata writes still require optimistic locking —
  see Fix r.Update to r.Patch plus regression guard in MCPServer controller #4767 (tracking) / Patch MCPServer spec instead of Update #4914 (MCPServer migration).
• The helper does not make every multi-writer pattern safe. The doc
  comment's Caller contract spells out what it cannot defend against:
  Conditions-array writers must be the sole owner of the entire array
  (merge-patch replaces arrays wholesale for CRDs; +listType=map is
  only honored by strategic-merge-patch), and a scalar re-computed from
  a stale snapshot that differs from the live value will clobber a
  concurrent writer.
• Operational safeguards: no-op mutations skip the wire call to avoid
  steady-state PATCH noise on fast-requeue reconcilers, and a nil obj
  returns a descriptive error instead of panicking in the .(T) assertion.
• Codified checklist for new call sites added to .claude/rules/operator.md.
• Pure addition — no existing call-site touched in this PR. Migration of
  the existing r.Status().Update call sites will follow.

Relates to #4633

Type of change

• Refactoring (no behavior change)

Test plan

• Unit tests (task test)
• Linting (task lint-fix)

Tests in cmd/thv-operator/pkg/controllerutil/status_test.go cover:

• Happy path: mutation applied in-memory AND reflected in the patch body.
• DeepCopy isolation: regression guard for snapshot aliasing.
• No-op mutate: helper skips the wire call when the diff is {}.
• Disjoint-writer preservation: stale snapshot + second writer on a different
  scalar field; fresh Get confirms both the mutated field and the second
  writer's fields survive.
• Stale snapshot clobbers conditions from another writer: guards the
  documented Caller contract (distinct condition types do not survive a
  stale writer's merge patch, so callers must own the entire array).
• Stale scalar computation: re-assigning the read value is a wire-level
  no-op (concurrent writer preserved); assigning a differing value
  overwrites live state.
• Nil obj rejected with a descriptive error, no PATCH issued.
• Error propagation: apiserver failure returned unchanged for requeue.

Does this introduce a user-facing change?

No.

Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 85.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.00%. Comparing base (a543b35) to head (2987cb0).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
cmd/thv-operator/pkg/controllerutil/status.go	85.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4942      +/-   ##
==========================================
- Coverage   69.00%   69.00%   -0.01%     
==========================================
  Files         552      553       +1     
  Lines       72996    73016      +20     
==========================================
+ Hits        50370    50383      +13     
- Misses      19628    19633       +5     
- Partials     2998     3000       +2     

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[ChrisJBurns]

Multi-Agent Consensus Review

Agents consulted: kubernetes-expert, go-expert-developer, code-reviewer, toolhive-expert

Consensus Summary

#	Finding	Consensus	Severity	Action
1	Conditions-array doc claim is only true when writers Get before mutating	8/10	HIGH	Clarify doc
2	Comment references non-existent mcpserver_spec_patch_test.go	10/10	MEDIUM	Fix / remove
3	\#4767 referenced as if spec-side helper exists	8/10	MEDIUM	Clarify doc
4	Typed-nil obj triggers raw panic	7/10	MEDIUM	Document or guard
5	Stale-snapshot scalar-field clobber — doc gap	7/10	MEDIUM	Document contract
6	No-op mutate still PATCHes	9/10	LOW	One-line doc note
7	Envtest variant preferred over fake-client for merge-patch semantics	7/10	LOW	Follow-up test

Overall

The helper itself is correct, small (48 lines), well-tested, and solves a real problem from #4633. The DeepCopyObject().(T) pattern, the decision to skip optimistic locking for status, the test structure (parallel, wire-body assertions, disjoint-writer regression), and the package placement are all sound. Four specialized agents reviewed the change and none disputed the fundamental design.

The findings cluster around documentation precision rather than the code behavior. The HIGH finding is the most important: the doc claim that "Conditions-array overlap stays safe via the per-condition idiom" is load-bearing on an invariant the helper does not enforce. client.MergeFrom produces RFC 7396 merge patches (confirmed in controller-runtime@v0.23.3/pkg/client/patch.go → jsonpatch.CreateMergePatch) which REPLACE arrays wholesale — the +listType=map marker on Conditions is only honored by strategic-merge-patch, which CRDs don't support. So the claim is true in practice only if every writer Gets the live object immediately before calling the helper. Worth stating that precondition explicitly so future call-site migrations don't take the doc at face value.

The two cross-references (mcpserver_spec_patch_test.go and \#4767 "spec-side rationale") point at code that does not yet exist in the repo. \#4767 is an open tracking issue, not a landed pattern, and there is no spec-patch-recording test file anywhere. These are fixable with a one-line edit each — either remove the forward references or rephrase to make clear they describe planned follow-up work.

None of these block merge in my view; a follow-up or amended commit covers them. The migration PRs that land on top of this helper will benefit from tighter documentation here.

Documentation

Per CLAUDE.md, architecture doc sync was considered. This is an internal helper with no user-facing or architecture-doc surface — docs/arch/ does not need updates. The in-package doc.go was correctly updated.

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