Remove v1 merge_insert code path

[wjones127]

What to build

Once all v1 fallback cases are migrated to v2, delete the v1 code:

• Merger struct and its methods
• The v1 branch in execute_uncommitted_impl
• can_use_create_plan gate (all operations now use v2)
• Helpers only used by v1 (e.g. parts of create_joined_stream not reused)
• Tests specific to v1 behavior or the v1/v2 fallback logic

See milestone description for full PRD.

Acceptance criteria

• Merger struct deleted
• can_use_create_plan removed
• v1 branch in execute_uncommitted_impl removed
• All existing merge_insert tests pass
• No dead code remains from v1 path

Blocked by

• Support WhenMatched::DoNothing on v2 merge_insert path #6441 (DoNothing on v2)
• Support partial schema upsert on v2 merge_insert path #6442 (partial schema on v2)
• Implement scalar-indexed joins on v2 merge_insert path #6444 (indexed joins on v2)

User stories addressed

• 4: single merge_insert code path