feat: Add output schema to select readonly tools under opt-in feature flag

[RossTarrant]

Summary

Adds feature-gated MCP outputSchema for selected read-only tools.

Why

Output schemas allow LLMs to anticipate the response from a tool. This proves useful to Code Mode, which needs machine-readable MCP responses for multi-step workflows.

What changed

• Added the output_schemas feature flag and inventory registration support for gated tool output schemas.
• Added minimal structured response types and schema inference helpers.
• Added structured outputs for selected context, issue, label, pull request, repository, and search tools.
• Preserved existing text content responses for back-compat when structured output is enabled.
• Added tests for feature-flag resolution, schema registration, toolsnap handling, and structured content gating.

MCP impact

• No tool or API changes
• Tool schema or behavior changed
  • When output_schemas is enabled, selected tools include outputSchema in tool definitions and return matching structuredContent.
• New tool added

Prompts tested (tool changes only)

• "List open issues in github/github-mcp-server"
• "Search pull requests in github/github-mcp-server about output schemas"
• "Search code in github/github-mcp-server for StructuredContent"
• "List recent releases for github/github-mcp-server"

Security / limits

• No security or limits impact
• Auth / permissions considered
  • Existing tool scopes and permissions are unchanged.
• Data exposure, filtering, or token/size limits considered
  • Structured payloads are feature-gated and use minimal response shapes while keeping legacy content populated.

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

Lint & tests

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

Docs

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

[Copilot]

Pull request overview

Adds opt-in MCP outputSchema support for selected read-only GitHub tools, with matching gated structuredContent responses while preserving legacy text responses.

Changes:

• Adds output_schemas feature flag plumbing and registration-time output schema attachment.
• Introduces schema inference/result helpers plus minimal response wrapper types.
• Adds structured outputs for selected context, issue, label, PR, repository, release, and search tools, with tests and toolsnap normalization updates.

Show a summary per file

File	Description
pkg/inventory/server_tool.go	Adds OutputSchema, registration options, and gated schema attachment.
pkg/inventory/registry.go	Enables output schema registration based on feature flag.
pkg/inventory/registry_test.go	Tests feature-gated registration behavior.
pkg/inventory/builder.go	Adds inventory-side output schema feature flag constant.
pkg/http/middleware/cors.go	Adds a new allowed CORS request header.
pkg/github/tools_validation_test.go	Validates expected tools have gated output schemas.
pkg/github/search.go	Adds structured outputs/schemas to search tools.
pkg/github/search_utils.go	Passes dependencies to shared search handler for feature-gated structured output.
pkg/github/repositories.go	Adds structured outputs/schemas to selected repository/release tools.
pkg/github/pullrequests.go	Adds structured outputs/schemas to PR listing/search tools.
pkg/github/output_schema.go	Adds output schema inference and structured text result helpers.
pkg/github/output_schema_test.go	Tests schema inference and structured content gating.
pkg/github/minimal_types.go	Adds minimal response wrapper/result types and converters.
pkg/github/labels.go	Adds structured output/schema for get_label.
pkg/github/issues.go	Adds structured outputs/schemas to selected issue tools.
pkg/github/feature_flags.go	Adds output_schemas to allowed feature flags.
pkg/github/feature_flags_test.go	Tests explicit output schema feature resolution.
pkg/github/context_tools.go	Adds structured outputs/schemas to selected context tools.
internal/toolsnaps/toolsnaps.go	Normalizes snapshots and strips gated outputSchema.
internal/toolsnaps/toolsnaps_test.go	Tests output schema stripping in toolsnap normalization.

Copilot's findings

• Files reviewed: 20/20 changed files
• Comments generated: 2