chore(azure): simplify calculation for getMergeMethod

[jamietanna]

Changes

Instead of the complicated check we were previously performing which:

• recovered from errors if policyConfigurations wasn't undefined, or
  other errors occurred
• trimmed the field names and checked if they were one of the
  GitPullRequestMergeStrategy options

We can instead rewrite this with stronger types and compile time checks
to confirm that the mapping from a policy configuration matches the
different GitPullRequestMergeStrategys.

This provides a clearer intent for what this code is doing.

This changes behaviour slightly - previously, the first JSON key defined
that was a GitPullRequestMergeStrategy would be used (which is JSON
implementation specific), whereas now it is a defined order.

We also add coverage for the new behaviour.

Context

Please select one of the below:

• This closes an existing Issue: Closes #
• This doesn't close an Issue, but I accept the risk that this PR may be closed if maintainers disagree with its opening or implementation

AI assistance disclosure

Did you use AI tools to create any part of this pull request?

Please select one option and, if yes, briefly describe how AI was used (e.g., code, tests, docs) and which tool(s) you used.

• No — I did not use AI for this contribution.
• Yes — minimal assistance (e.g., IDE autocomplete, small code completions, grammar fixes).
• Yes — substantive assistance (AI generated non‑trivial portions of code, tests, or documentation).
• Yes — other (please describe):

Documentation (please check one with an [x])

• I have updated the documentation, or
• No documentation update is required

How I've tested my work (please select one)

I have verified these changes via:

• Code inspection only, or
• Newly added/modified unit tests, or
• No unit tests but ran on a real repository, or
• Both unit tests + ran on a real repository

The public repository:

[jamietanna]

Wondering if we should do it as a "fix" to:

• get it released
• indicate that there may be functionality change

?

[viceice]

otherwise LGTM

[github-actions]

🎉 This PR is included in version 43.141.0 🎉

The release is available on:

• GitHub release
• 43.141.0

Your semantic-release bot 📦🚀