fix(core): preserve shared references in JSON serialization

[Oscar-Williams]

Summary

Fixes #29406 by treating only objects on the active JSON recursion path as circular. Repeated OpenTelemetry arrays in file exports now retain their values instead of becoming [Circular].

Details

• Replaced the process-wide WeakSet with active ancestor-path tracking.
• Kept [Circular] for genuine ancestor cycles.
• Added regressions for generic shared references, span endTime/duration, and metric endTime/histogram boundaries.

Related Issues

Fixes #29406

How to Validate

• npm test -w @google/gemini-cli-core -- src/utils/safeJsonStringify.test.ts src/telemetry/file-exporters.test.ts (20 passed)
• npm run typecheck -w @google/gemini-cli-core
• npm run lint -w @google/gemini-cli-core
• npx prettier --check packages/core/src/telemetry/file-exporters.test.ts packages/core/src/utils/safeJsonStringify.test.ts packages/core/src/utils/safeJsonStringify.ts
• git diff --check

The repository preflight reached dependency installation, formatting, build, ESLint, actionlint, shellcheck, and yamllint; the Windows run then stopped because grep is unavailable. The full test suite reached all workspaces and passed the changed tests, with unrelated Windows trust, symlink, PTY, and sandbox integration failures.

Pre-Merge Checklist

• Added/updated tests
• No breaking changes
• Targeted tests, typecheck, lint, formatting, and diff checks pass

AI assistance was used for repository research, implementation, validation, and PR drafting; the patch was checked against the repository requirements.

[github-actions]

📊 PR Size: size/M

• Lines changed: 118
• Additions: +109
• Deletions: -9
• Files changed: 3

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request improves the JSON serialization utility by refining how circular references are detected. By shifting from a process-wide tracking approach to one that only considers the current recursion path, the implementation now correctly handles shared object references that are not part of a recursive cycle. This change ensures that telemetry data, such as OpenTelemetry spans and metrics, maintains its integrity during serialization rather than being prematurely truncated.

Highlights

• Circular Reference Logic: Replaced the global WeakSet-based circular reference detection with an active ancestor-path tracking mechanism to ensure only true recursive cycles are marked as [Circular].
• Data Integrity: Fixed an issue where shared references in OpenTelemetry spans and metrics were incorrectly serialized as [Circular], ensuring values are preserved during JSON export.
• Regression Testing: Added comprehensive test cases for shared span values, metric end times, histogram boundaries, and generic shared object references.

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[gemini-code-assist]

Code Review

This pull request updates the safeJsonStringify utility to correctly handle repeated, non-circular object references. Instead of using a WeakSet which flags any previously seen object as circular, the implementation now tracks the active ancestor path using an array and pops elements as the traversal backtracks. This ensures that shared references (such as shared end times or histogram boundaries in telemetry spans and metrics) are preserved in the serialized JSON output rather than being incorrectly replaced with [Circular]. Corresponding unit tests have been added to verify this behavior for both the utility and the file exporters. I have no feedback to provide as the changes are well-implemented and fully tested.