fix: preserve JSON-looking strings in body mutations

[git-jxj]

Description

A bodyMutation.set value such as 1.0.0, +1, or [abc] passes the JSON-looking heuristic and is inserted unquoted, corrupting an otherwise valid request body. Use the existing gjson.Valid validator so non-JSON values follow the existing string fallback. Valid JSON literals, objects, and arrays retain their types.

Regression tests exercise BodyMutator.Mutate with malformed numbers, objects, arrays, escape sequences, empty strings, and valid JSON values.

Related Issues/PRs (if applicable)

None.

Special notes for reviewers (if applicable)

• make precommit: passed, including generated API files, lint, format, and Helm checks.
• make test: passes with TestStartConfigBundleWatcher and TestNewRunOpts_Errors skipped. Both failures reproduce on unchanged main; the permission tests also pass in a subprocess without DAC bypass capabilities.
• A local HTTP harness returns 400 before the fix and 204 after it for the same version-string mutation.

Codex assisted with the implementation and regression tests.

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[git-jxj]

The workflows for current head b1b18aa656a2 are still awaiting maintainer approval: Precommit Check, CodeQL, Build and Test. Could a maintainer approve these current-head runs so CI validation can start?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[git-jxj]

Merged main at ca3e5166 in 75f12ff9 to pick up the existing MCP benchmark fix in #2856.

On Linux, the full make test-data-plane-mcp (including all four benchmarks) and make precommit pass. The complete unit run has one failure, TestStartConfigBundleWatcher; the same assertion reproduces on unchanged main ca3e5166.

The previous inference job timed out waiting for the secondary Gateway parent; its final InferencePool status contained only the primary parent. That controller/conformance source is identical on main, so this still needs separate CI validation.

Could a maintainer approve Build and Test, Precommit, and CodeQL for 75f12ff9? All three new runs are action_required. macOS MCP and inference conformance have not yet run on this head.