Enable fields param in Insiders mode

[tommaso-moro]

Summary

Enable the fields_param feature flag automatically in Insiders mode while preserving direct opt-in through the existing feature allowlist.

Why

The response-filtering parameter introduced in #2775 and #2810 is experimental, so it should be available to Insiders users without changing the standard server's default tool schemas.

No linked issue.

What changed

• Added FeatureFlagFieldsParam to InsidersFeatureFlags.
• Added coverage for default-off, direct opt-in, and Insiders resolution behavior.
• Regenerated the Insiders feature documentation with the affected tool schema deltas.

MCP impact

• No tool or API changes
• Tool schema or behavior changed - Insiders mode now advertises the existing fields parameter on supported tools.
• New tool added

Prompts tested (tool changes only)

• N/A - existing tool behavior is unchanged; feature flag resolution is covered by automated tests.

Security / limits

• No security or limits impact
• Auth / permissions considered
• Data exposure, filtering, or token/size limits considered - filtering behavior is unchanged; this only enables the existing response-size feature for Insiders.

Tool renaming

• I am renaming tools as part of this PR (e.g. a part of a consolidation effort)
  • I have added the new tool aliases in deprecated_tool_aliases.go
• I am not renaming tools as part of this PR

Note: if you're renaming tools, you must add the tool aliases. For more information on how to do so, please refer to the official docs.

Lint & tests

• Linted locally with ./script/lint
• Tested locally with ./script/test

Docs

• Not needed
• Updated (README / docs / examples)

[Copilot]

Pull request overview

This PR expands Insiders mode feature-flag resolution so the existing fields_param-gated tool variants are automatically enabled for Insiders users, while keeping the existing direct opt-in path via the allowlist for non-Insiders.

Changes:

• Added FeatureFlagFieldsParam to InsidersFeatureFlags so Insiders mode advertises the fields parameter on supported tools.
• Added/updated unit tests covering default-off, direct opt-in, and Insiders behavior.
• Regenerated docs/insiders-features.md to include the fields_param tool schema deltas.

Show a summary per file

File	Description
pkg/github/feature_flags.go	Enables fields_param automatically in Insiders mode by adding it to InsidersFeatureFlags.
pkg/github/feature_flags_test.go	Adds tests for default-off and direct/Insiders enablement behavior.
docs/insiders-features.md	Updates generated Insiders documentation to show the fields_param-enabled tool schema deltas.

Review details

• Files reviewed: 3/3 changed files
• Comments generated: 1
• Review effort level: Low