#6228 fix(desktop): respect repository merge settings in PR merge menus

Design

Both pull request merge menus use the same pure merge-method selection logic and the same capability mapping.

• @superset/shared/github-merge-methods owns the supported method set, REST-to-canonical capability normalization, and explicit-disabled check.
• The renderer uses that capability check while filtering methods and keeps the viewer default first when it is enabled.
• v2 reads repository capabilities through github.getRepo; v1 reads the equivalent fields through gh repo view, resolving fork worktrees to their upstream repository.
• Both merge mutations reject a method only when GitHub explicitly reports it as disabled. Missing, incomplete, or failed settings lookups remain fail-open, preserving the existing merge behavior.
• No database schema changes are required.

Validation

• Shared capability tests: 2 passing
• Merge menu tests: 4 passing
• v1 merge router tests: 3 passing
• GitHub router integration tests: 10 passing
• Existing v1 merge utility tests: 3 passing
• Biome lint and format checks: passing
• Shared and host-service typechecks: passing
• Desktop typecheck remains blocked by the workspace's missing @sentry/electron/main and @sentry/electron/renderer modules.

Closes #6185

Summary by CodeRabbit

• New Features

  • Pull request merge menus now reflect the repository’s enabled merge methods.
  • The viewer’s preferred merge method is prioritized when available.
  • Merge method labels and selections are consistent across pull request interfaces.
  • Unavailable repository settings fall back to standard merge options.

• Bug Fixes

  • Disabled merge methods are no longer displayed or accepted for merging.

• Tests

  • Added coverage for repository-specific methods, defaults, fallbacks, and disabled options.