refactor(control-plane): split handlers/nodes.go into 5 focused files

[kiranannadatha8]

Closes #416

Summary

Splits control-plane/internal/handlers/nodes.go (1753 LOC, 22 funcs, 3 types, 2 package vars — mixing registration, heartbeat, list/detail, status lifecycle, and URL-discovery helpers) into 5 focused files within the same handlers package. Pure file moves: no renames, no behavior change.

File layout

File	LOC	Contents
nodes_register.go	950	RegisterNodeHandler, RegisterServerlessAgentHandler, readCloser + validate, and all URL/candidate helpers (validateCallbackURL, extractPortFromURL, gatherCallbackCandidates, normalizeCandidate, resolveCallbackCandidates, normalizeServerlessDiscoveryURL, isServerlessDiscoveryHostAllowed)
nodes_heartbeat.go	298	HeartbeatHandler, HeartbeatCache, CachedNodeData, heartbeatCache/dbUpdateThreshold vars, shouldUpdateDatabase, processHeartbeatAsync
nodes_status.go	460	Lifecycle/status: UpdateLifecycleStatusHandler, GetNodeStatusHandler, RefreshNodeStatusHandler, BulkNodeStatusHandler, RefreshAllNodeStatusHandler, StartNodeHandler, StopNodeHandler
nodes_list.go	57	ListNodesHandler
nodes_detail.go	36	GetNodeHandler

Before: 1 file × 1753 LOC → After: 5 files summing to 1801 LOC (+48 from 4 extra package/import blocks + gofmt of a pre-existing stray indentation in the register section).

The old nodes.go is deleted. The issue allows nodes.go to remain for "tiny shared helpers only" — after the split there are no shared helpers left that would otherwise be homeless, so an empty nodes.go would be noise. Git detects the rename of nodes.go → nodes_register.go (54% similarity), which keeps the diff reviewable.

Why this split

The issue proposed: nodes_register.go / nodes_heartbeat.go / nodes_list.go / nodes_detail.go / (nodes_tags.go if tag approval lives here) / nodes.go (shared helpers).

Tag approval handlers are in handlers/admin/tag_approval.go, not this file, so no nodes_tags.go. The 7 status/lifecycle handlers (UpdateLifecycleStatusHandler, GetNodeStatusHandler, RefreshNodeStatusHandler, BulkNodeStatusHandler, RefreshAllNodeStatusHandler, StartNodeHandler, StopNodeHandler) form a natural group and don't fit into detail/list, so they live together in nodes_status.go. Open to re-splitting if the reviewer prefers a different boundary.

Guardrails respected

• No renames — diff <(grep '^(func|type|var|const) ' original | sort) <(cat new_files | grep '^(func|type|var|const) ' | sort) is empty
• No behavior change — function bodies copied verbatim; gofmt'd per Go style
• Existing tests unchanged — nodes_discovery_test.go, nodes_rest_test.go, nodes_test.go, nodes_serverless_revocation_test.go, and the coverage_*_test.go files that poke heartbeatCache/processHeartbeatAsync/readCloser/CachedNodeData by name all resolve unchanged via same-package visibility

Test plan

• cd control-plane && go build ./... → green
• cd control-plane && go test ./internal/handlers/... -count=1 → ok across all 5 handler sub-packages (handlers, handlers/admin, handlers/agentic, handlers/connector, handlers/ui)
• cd control-plane && go vet ./... → clean
• gofmt -l control-plane/internal/handlers/nodes*.go → clean
• cd control-plane && go test ./... -count=1 → handlers green; only pre-existing failures in internal/application, internal/core/services, internal/server (TestGenerateAgentFieldServerID, TestDevServicePortHelpers*, TestGenerateAgentFieldServerIDFallsBackWhenGetwdFails, TestStartAdminGRPCServerReturnsListenError). These all reproduce on a fresh clone of main — unrelated to this PR
• CI green

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