[backport] Fix delete:true on issue fields — REST DELETE fallback (upstream main)

[gastrodon]

Summary

Fixes a silent no-op bug: when deleting the last (or only) issue field value, go-github's omitempty tag on IssueRequest.IssueFieldValues strips the empty slice from the PATCH body. The GitHub REST endpoint's if data.include?(ISSUE_FIELD_VALUES) guard then skips the whole block, leaving the field unchanged. The MCP tool reported success.

Fix: detect the empty-after-filter case in UpdateIssue and fire individual DELETE /repos/{owner}/{repo}/issues/{number}/issue-field-values/{field_id} requests per field as fallback. The DELETE endpoint is idempotent (404 for already-absent fields filtered out), and partial failures are accumulated and reported with per-field detail rather than short-circuiting.

New tests in pkg/github/issues_delete_test.go:

• Test_IssueRequest_EmptyFieldValues_OmittedByJSON — pins the omitempty contract
• The 1-of-1 fallback path
• The N-1 set-semantics path (non-empty kept list → PATCH alone)
• Test_UpdateIssue_DeleteAbsentFieldIsNoOp — already-cleared field is silent no-op
• Test_UpdateIssue_DeleteFallbackContinuesOnPartialFailure — partial failures aggregated

Upstream reference

• version: upstream main (post v1.4.0, unreleased)
• commits: github@29634da
• PR: Fix delete:true on issue fields by calling deleteIssueFieldValue mutation github/github-mcp-server#2755

Fork conflict

None. The fork does not have issue field write tools; this is in issues.go in the shared area.

Context

Agents using issue_write with delete: true on issue field values received a success response but the field was never cleared. This was invisible without checking the field state after the call, making it a hard-to-debug data integrity bug in automated workflows.

[gastrodon]

Unified Changelog — upstream main since v1.4.0

See #10 for the full changelog. Summary:

• [backport] OAuth 2.1 stdio login (upstream main) [story] #13 (story): OAuth 2.1 stdio login → sub-issues [backport] OAuth 2.1 core library — PKCE + device flows (upstream main) #10 [backport] Wire OAuth 2.1 login into stdio server (upstream main) #11 [backport] OAuth build-time credentials + docs (upstream main) #12
• [backport] Issue dependency read/write tools (upstream main) #14: Issue dependency read/write tools
• [backport] get_parent method for issue_read (upstream main) #15: get_parent for issue_read
• [backport] Promote issue fields to default, remove legacy issue_write methods (upstream main) #16: Promote issue fields, remove legacy issue_write
• [backport] Remove show_ui parameter, centralize show/defer decision (upstream main) #17: Remove show_ui, centralize show/defer
• [backport] Fix delete:true on issue fields — REST DELETE fallback (upstream main) #18: Fix delete:true on issue fields
• [backport] Bound ui_get pagination to prevent latency spikes (upstream main) #19: Bound ui_get pagination
• [backport] Fix issue_write MCP App form gating for labels/assignees/milestone/type (upstream main) #20: Fix issue_write form gating
• [backport] Validate required params in add_comment_to_pending_review (upstream main) #21: Validate required params in add_comment_to_pending_review

[gastrodon]

No-op. The fork already has pkg/github/issues_delete_test.go with all five test functions covering the omitempty contract, the 1-of-1 fallback, the N-1 set-semantics path, delete-absent-field no-op, and partial failure aggregation. The REST DELETE fallback logic is present in pkg/github/issues.go.