Fix auto-merge API to merge immediately when no pipeline exists

What does this MR do and why?

When the merge API is called with auto_merge=true (or the deprecated merge_when_pipeline_succeeds=true) but no auto-merge strategy is available and no pipeline exists for the MR, the API now falls back to merging immediately if the MR is mergeable, instead of returning HTTP 405 Method Not Allowed.

Problem

Projects that skip CI pipelines for certain MRs via workflow:rules (e.g., automated cleanup MRs) cannot use merge_when_pipeline_succeeds: true because:

1. The workflow rule suppresses the pipeline entirely
2. No auto-merge strategy is available (nothing to wait for)
3. diff_head_pipeline_success? returns false (no pipeline exists)
4. The API returns 405 instead of just merging the already-mergeable MR

Solution

Extract a no_pipeline_blocking_merge? helper in the API layer that checks whether the pipeline has succeeded or no pipeline exists (and none is being created). This allows the immediate-merge fallback path in execute_merge_with_fix to handle the no-pipeline case.

This is intentionally scoped to only the no-pipeline case — a failed pipeline still returns 405, preserving existing behavior and API/UI parity.

Scenario	Before	After
No pipeline, CI pass not required, MR mergeable	405	Merges immediately
No pipeline, CI pass required	405	Auto-merge enabled (waits for pipeline)
No pipeline, pipeline being created	405	Auto-merge enabled (waits for pipeline)
Pipeline failed, CI pass not required	405	405 (preserved)
Pipeline succeeded, MR mergeable	Merges	Merges (no change)
Pipeline running	Auto-merge enabled	Auto-merge enabled (no change)

Related

• Fixes #355455
• Follow-up from !197358 (merged)
• Supersedes !218845 (closed)

MR acceptance checklist

• Tests added/updated
• Conforms to the merge request performance guidelines
• Conforms to the style guides