use the REST PATCH endpoint if updating rationale or suggestion

[boazreicher]

Follow-up to #2557.

Summary

When set_issue_fields is called with a rationale or is_suggestion, route the request through the REST PATCH /repos/{owner}/{repo}/issues/{issue_number} endpoint instead of the GraphQL setIssueFieldValue mutation, since GraphQL does not currently support those parameters.

Why

#2557 added rationale and is_suggestion to the set_issue_fields tool, but the GraphQL mutation does not accept those inputs. The REST update-issue endpoint does (the same way update_issue_labels already uses it for labels with rationale).

What changed

• pkg/github/issues_granular.go:
  • In GranularSetIssueFields, set a needsREST flag when any field has a non-empty rationale or is_suggestion: true. Existing GraphQL parsing/dispatch is unchanged otherwise.
  • When needsREST is set, dispatch via a new setIssueFieldsViaREST helper:
    • Calls the existing fetchIssueFields to resolve each field's integer database ID and (for single-select fields) option name.
    • PATCHes the issue with body shape { "issue_field_values": [ { "field_id": <int>, "value": "<string>", "rationale": "...", "suggest": true } ] }.
    • Returns an error if delete: true is combined with rationale/is_suggestion (not supported on the REST shape).
• pkg/github/granular_tools_test.go: updated the two rationale/suggest test cases to mock the new REST flow (GraphQL field-defs query + REST PATCH) instead of the GraphQL mutation.

No tool schema changes.

MCP impact

• No tool or API changes
• Tool schema or behavior changed
• New tool added

The tool's input/output schema is unchanged; only the underlying transport changes when rationale/is_suggestion is set.

Prompts tested (tool changes only)

N/A — input schema unchanged.

Security / limits

• No security or limits impact

Tool renaming

• I am renaming tools as part of this PR (e.g. a part of a consolidation effort)
• 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

This PR updates the granular set_issue_fields tool to route requests through the REST Issues PATCH endpoint when rationale and/or is_suggestion are provided, since those attributes aren’t supported by the GraphQL setIssueFieldValue mutation.

Changes:

• Detects when a request includes rationale/is_suggestion and switches execution from GraphQL to REST.
• Adds a REST fallback implementation that resolves field database IDs (and single-select option names) via GraphQL before PATCHing the issue.
• Updates granular tool tests to validate the REST wire format for rationale/suggestion.

Show a summary per file

File	Description
pkg/github/issues_granular.go	Adds needsREST routing plus a REST PATCH implementation for issue field updates with rationale/suggestion.
pkg/github/granular_tools_test.go	Updates tests to mock GraphQL field-definition lookup and assert REST PATCH payloads for rationale/suggestion cases.

Copilot's findings

• Files reviewed: 2/2 changed files
• Comments generated: 7