feat: BB client gen (for cloud + server) + merge strategy support

[fuskovic]

• Replaces the hand-rolled bbClient HTTP client in the Bitbucket git provider with a typed client generated from the Bitbucket Cloud OpenAPI spec via oapi-codegen v2.
• Removes the github.com/ktrysmt/go-bitbucket dependency.
• Adds a trimmed Bitbucket OAS3 spec (spec/bitbucket.gen.json) containing only the 4 endpoints used by the provider
• Adds acodegen-bitbucket-client Makefile target (backed by oapi-codegen pinned in hack/tools/) so the generated client can be regenerated in one command.
• Adds BitBucket merge method support.
• Updates git-merge-pr docs to list the supported Bitbucket merge methods now that the generated enum makes them explicit.

Closes: #6023
Closes: #6022
Closes: #6020

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[codecov]

Codecov Report

❌ Patch coverage is 90.63670% with 50 lines in your changes missing coverage. Please review.
✅ Project coverage is 57.91%. Comparing base (91a058b) to head (4ae10c1).

Files with missing lines	Patch %	Lines
pkg/gitprovider/bitbucket/cloud/provider.go	84.34%	30 Missing and 14 partials ⚠️
pkg/gitprovider/bitbucket/datacenter/provider.go	97.62%	4 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6128      +/-   ##
==========================================
+ Coverage   57.67%   57.91%   +0.23%     
==========================================
  Files         474      475       +1     
  Lines       40506    40822     +316     
==========================================
+ Hits        23362    23642     +280     
- Misses      15749    15775      +26     
- Partials     1395     1405      +10     

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[krancour]

We were already using go-swagger for generating the client for our own APIs. I'm open to using something else, such as this, but ideally we wouldn't use two different tools for the same purpose.

Did you happen to try either generating the BB client using go-swagger or, inversely, try generating our own client using oapi-codegen?

[fuskovic]

The two tools target different spec versions and are not interchangeable here.go-swagger only supports Swagger 2.0 (OAS2) specs. The Bitbucket Cloud API spec is OpenAPI 3.0 (OAS3),

[krancour]

I think there are some things to discuss here still, but I am really happy to see that generating a BB client and using that appears tenable.

[fuskovic]

@krancour so far we are able to:

1. Remove github.com/ktrysmt/go-bitbucket
2. Support Datacenter/Server
3. Support alternative merge strategies for both

This obviously needs a lot of end-to-end testing.

Per our offline discussion I have sub-packages:

• pkg/gitprovider/bitbucket/cloud
• pkg/gitprovider/bitbucket/datacenter

Implemented each with their own individual registration predicate.

[fuskovic]

I can take this out of draft and start end-to-end testing unless there's something else we still want to discuss?