fix(openai): reject oneOf/allOf and validate $defs/$ref before enabling strict mode (#4106)

[aheritier]

🤖 Automated implementer agent — this comment was posted by the implementer bot from Docker Agentic Platform, not by a human developer

Closes #4106.

Problem

isStrictCompatible/hasIncompatibleNode only checked additionalProperties, never rejected oneOf/allOf/other composition keywords OpenAI Structured Outputs doesn't support, and neither it nor the schema normalizer (walkSchema) traversed $defs/definitions. A tool whose schema used oneOf (the issue's repro) or had an unnormalized $defs entry was sent to OpenAI with strict: true and rejected with HTTP 400, killing the whole request.

A third, related defect found during design: the shared pkg/tools/schema.go ensurePropertyTypes helper injected type: "object" onto $ref nodes, which downstream normalization then widened with additionalProperties/nullable-type — producing a $ref node with sibling keywords, which OpenAI strict mode rejects.

Fix

• pkg/model/provider/openai/schema.go: shared childSchemas iterator (iter.Seq) used by both the compatibility check and the normalizer, covering properties/patternProperties/$defs/definitions/anyOf/oneOf/allOf/prefixItems/items/additionalProperties/not/if/then/else/contains/propertyNames.
• hasIncompatibleNode rejects oneOf, allOf, not, if, then, else, dependentRequired, dependentSchemas outright, and validates $ref nodes via a small local JSON-pointer resolver (string, local #/#/..., resolvable, no sibling keywords).
• $ref nodes are treated as leaves by makeAllRequired/ensureTypeFields (no type/additionalProperties injection). A newly-required (optional) $ref property is wrapped as {"anyOf": [<$ref>, {"type": "null"}]}, OpenAI's documented optional-reference pattern — with ensureTypeFields also skipping anyOf/oneOf/allOf nodes so this wrapper doesn't get a conflicting type sibling (a real bug an earlier draft of this PR had, caught in review).
• pkg/tools/schema.go's ensurePropertyTypes skips $ref nodes (the root cause of the pollution above).

Testing

• New test coverage in pkg/model/provider/openai/schema_test.go (T1–T14 from the design doc, plus an invariant test: normalizing a strict-compatible schema must never make it incompatible) and pkg/tools/schema_test.go.
• task build, task test, task lint all green (full suite, including go mod tidy --diff and the project's custom go run ./lint . cops).
• Not automated: a live-API check of whether OpenAI accepts a $ref node with a description sibling under strict: true (would relax the current conservative "no sibling keywords" rule to "no siblings other than description", per the openai-python SDK precedent this design follows). No OpenAI API key was available in this environment. Documented as a follow-up rather than blocking this fix.

Out of scope

Structured Outputs size limits, root-level anyOf rejection, anyOf-typed optional property nullable-widening (pre-existing, unrelated gap), and inlining $ref (rejected approach — would break recursive schemas).