feat: gen BB-cloud client + alternative merge-strategy support

[fuskovic]

#6128 was blocked on BB datacenter/server end-to-end testing so I decided to move them into a separate PR to unblock progress on non-server/datacenter logic.

• Closes proposal: generate a Bitbucket Cloud API client from the official OpenAPI spec #6023 and removes the github.com/ktrysmt/go-bitbucket dependency.
• Closes feat: support alternatively specified merge strategies for bitbucket #6022

Demo

The promotion workflow is configured to:

• Make two commits on a new branch.
• Each commit adds a new file.
• git-merge-pr is configured to use the squash merge strategy.
• Demo shows the 2 file updates are consolidated into one commit on merge.

squash.mov

Test repo was using a merge_commit default merge strategy.

This means the merge method we configured was prioritized over the default (as expected) :

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[codecov]

Codecov Report

❌ Patch coverage is 84.34164% with 44 lines in your changes missing coverage. Please review.
✅ Project coverage is 58.15%. Comparing base (6f12e62) to head (fca109c).
⚠️ Report is 51 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/gitprovider/bitbucket/cloud/provider.go	84.34%	30 Missing and 14 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6315      +/-   ##
==========================================
+ Coverage   57.90%   58.15%   +0.25%     
==========================================
  Files         496      500       +4     
  Lines       41495    41886     +391     
==========================================
+ Hits        24026    24358     +332     
- Misses      16020    16066      +46     
- Partials     1449     1462      +13     

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[hairyhum]

LGTM