Fix set_issue_fields mutation: use correct inline fragments for IssueFieldValue union

[iulia-b]

Summary

Fix the set_issue_fields MCP tool's GraphQL mutation: calls failed with:

Field 'name' doesn't exist on type 'IssueFieldDateValue'

Root Cause

The mutation response struct used a single inline fragment ... on IssueFieldDateValue with a Name field, but IssueFieldDateValue doesn't have a name field — only IssueFieldSingleSelectValue does.

Since GraphQL validates the entire document (including response selection sets) before executing any operation, the invalid fragment caused GraphQL to reject the entire request. The mutation never fired at all — no fields were ever set, regardless of input.

Fix

Replace the single broken inline fragment with correct fragments for all four IssueFieldValue union types:

• IssueFieldTextValue → value
• IssueFieldSingleSelectValue → name
• IssueFieldDateValue → value
• IssueFieldNumberValue → value

The response still uses MinimalResponse (issue ID + URL), so the additional fragments are purely for GraphQL validation correctness and future-proofing.

Testing

• All existing unit tests pass
• Updated test mock to match the corrected mutation struct

[Copilot]

Pull request overview

Fixes the set_issue_fields granular MCP tool’s GraphQL mutation selection set so it uses valid inline fragments for the IssueFieldValue union, preventing GraphQL from rejecting the request at validation time.

Changes:

• Updated the setIssueFieldValue mutation response struct to use correct inline fragments for all supported IssueFieldValue union members.
• Updated the corresponding GraphQL mock matcher in unit tests to reflect the corrected selection set.
• Minor formatting alignment in an HTTP handler test table.

Show a summary per file

File	Description
pkg/github/issues_granular.go	Fixes mutation response selection set by adding correct union inline fragments for issue field values.
pkg/github/granular_tools_test.go	Updates mocked mutation matcher struct to match the new inline fragments.
pkg/http/handler_test.go	Formatting-only alignment in the Content-Type handling test.

Copilot's findings

• Files reviewed: 2/3 changed files
• Comments generated: 1

[Copilot]

IssueFieldValues is requested in the mutation response but never read (the tool only marshals MinimalResponse from Issue). Consider removing IssueFieldValues from the selection set to reduce response payload and avoid having to maintain inline fragments for union validation; if you want to keep it, add a short comment explaining why it's intentionally unused.