Handle unsupported issueFieldValues enrichment in search_issues

[kerobbi]

Summary

Makes the custom field value enrichment in search_issues best-effort, so a search no longer fails when the server's GraphQL schema doesn't support the issueFieldValues field (e.g. GHES).

Why

Fixes #2831

What changed

• Made the follow-up GraphQL field value enrichment best-effort: a failure getting the GraphQL client or running the enrichment query is now recorded on the request context instead of failing the search
• search_issues returns its REST hits (with field_values omitted) when enrichment is unavailable, matching how issue_read get already treats the same enrichment
• Added Test_SearchIssues_FieldValuesEnrichmentUnsupported covering the unsupported-schema path

MCP impact

• Tool schema or behavior changed - search_issues now degrades gracefully instead of failing when field value enrichment is unsupported

Security / limits

• No security or limits impact

Tool renaming

• 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

[Copilot]

Pull request overview

This PR updates the search_issues tool’s custom-field enrichment to be best-effort so it no longer fails on GitHub Enterprise Server instances whose GraphQL schema does not support issueFieldValues (e.g., GHES 3.20.x). The search now returns REST results even when GraphQL enrichment is unavailable, aligning with the existing “best-effort enrichment” behavior in issue_read get.

Changes:

• Made search_issues GraphQL-based field_values enrichment non-fatal and record enrichment failures on the request context.
• Ensured search_issues returns REST hits with field_values omitted when enrichment cannot be performed.
• Added a unit test covering the unsupported-schema (issueFieldValues missing) path.

Show a summary per file

File	Description
pkg/github/issues.go	Makes search_issues field-value enrichment best-effort (do not fail the search when enrichment fails).
pkg/github/issues_test.go	Adds coverage for the unsupported GraphQL schema scenario to ensure REST results still return.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Low