fix(manager/swift): don't rewrite Package.resolved pins in unrelated packages

[fxwx23]

Changes

For exact (single-version) specs, only update a pin whose current version matches the version being upgraded from. This prevents the repo-wide Package.resolved sweep from rewriting a same-named transitive pin in an unrelated package to a version that package's constraints don't allow.

Context

Please select one of the following:

• This closes an existing Issue, Closes: Swift: updating one dependency rewrites unrelated transitive Package.resolved pins to incompatible versions #44312
• 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):

Used Claude (via Claude Code): Claude Opus 4.8 for the investigation, the fix design, and drafting the code/tests; Claude Sonnet 5 for applying the changes to the working tree and running the test/lint verification. All output was reviewed by a human before submission.

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: