fix: redact user content in the 7 passthrough endpoints

[Aias00]

Description

RedactSensitiveInfoFromRequest was only implemented for 4 of the 11 endpoint specs (ChatCompletions, Speech, Transcription, Translation); the other 7 (Completions, Embeddings, ImageGeneration, Responses, Messages, Rerank, Tokenize) were passthrough stubs returning the request unredacted. With debugLogEnabled && enableRedaction, those 7 endpoints logged raw user content (prompts, embedding inputs, images, conversation items, documents, PII) to the debug log despite redaction being requested — so enableRedaction was silently half-honored, which is arguably worse than not having it.

This implements real redaction for the 7 endpoints:

• Concrete string fields (Prompt, Query, Documents, Instructions, User) use RedactString.
• Chat messages (Embeddings OfChat, Tokenize OfChat) reuse the existing redactMessage helpers.
• Untyped-union fields (PromptUnion.Value, EmbeddingRequestInput.Value) and typed-union fields whose unmarshalers discriminate by a "type" key (MessageContent, SystemPrompt, ResponseNewParamsInputUnion, ResponsePromptParam) use a new redaction.RedactJSONTree that recursively redacts every string leaf in a JSON tree, preserving "type" discriminator values so the redacted tree still round-trips into the typed union.

Over-redaction of structural strings in debug logs is acceptable and safe; the goal is to never miss a sensitive nested string.

Related Issues/PRs (if applicable)

Fixes #2387

Special notes for reviewers (if applicable)

• The 4 already-implemented endpoints (ChatCompletions, Speech, Transcription, Translation) are left untouched; their careful typed redaction preserves tool names / schemas.
• redactUnionField marshals/unmarshals via pointers (&in / &out) so pointer-receiver MarshalJSON/UnmarshalJSON on the union types is actually invoked — passing a value T to json.Marshal would skip pointer-receiver methods and corrupt the union.
• RedactJSONTree preserves the value of a "type" map key (JSON-union discriminator) rather than redacting it; redacting "type" would make the redacted value fail to round-trip back into the typed union during redaction, and discriminator values carry no sensitive content.
• No call-site change: internal/extproc/processor_impl.go already dispatches to RedactSensitiveInfoFromRequest for every spec when redaction is on; the 7 stubs simply start doing real work.
• Tests assert that a known marker string does NOT appear in the redacted JSON, that a [REDACTED placeholder appears, and that the original request is not mutated.

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.97%. Comparing base (f9a03db) to head (b312200).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2428      +/-   ##
==========================================
+ Coverage   84.86%   84.97%   +0.10%     
==========================================
  Files         154      154              
  Lines       22419    22504      +85     
==========================================
+ Hits        19026    19122      +96     
+ Misses       2237     2225      -12     
- Partials     1156     1157       +1     

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Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated no new comments.

Comments suppressed due to low confidence (2)

internal/redaction/redaction.go:88

• RedactJSONTree currently preserves the value under the "type" key verbatim. If a request includes a non-string value for "type" (e.g. an object/array), this bypasses redaction for any nested string leaves under that value and also violates the "returns a new tree" claim by reusing the original reference. To keep union discriminator strings while still ensuring all string leaves are redacted, only preserve "type" when it is a string; otherwise recurse and redact/copy as usual.

			// Keys are structural (field/type names), not user data — keep them.
			// A "type" key holds a JSON-union discriminator value, not user data — keep it.
			if k == "type" {
				out[k] = e
				continue
			}
			out[k] = RedactJSONTree(e)
		}

internal/redaction/redaction_test.go:74

• Add a regression test to ensure that preserving the "type" key cannot be used to bypass redaction when its value is not a discriminator string (e.g., an object/array). This guards against accidental or malicious payloads leaking nested string content via the special-case.

	t.Run("type discriminator preserved", func(t *testing.T) {
		// "type" values are JSON-union discriminators, not user data — they must be
		// preserved so the redacted tree still round-trips into typed unions.
		in := map[string]any{
			"type":    "message",