fix(internal/librarian/golang): update version in snippet metadata

[JoeWang1127]

Replace placeholder with library version in snippet metadata file.

The logic is similar in go generator used in legacylibrarian:
https://github.com/googleapis/google-cloud-go/blob/b3409e6d7fbb5708e6be6000a1257a741c23b63b/internal/librariangen/module/module.go#L72

The file is read in one-go and modified in-place, which is not ideal if the file is too large. However, the metadata file is ~500 lines so this straightforward implementation should be suffice.

For #3617

[gemini-code-assist]

Code Review

This pull request introduces functionality to update a version placeholder in snippet metadata files. The implementation in generate.go uses filepath.Walk to find and update the files. A corresponding test is added in generate_test.go.

My review includes two main points:

1. A suggestion to improve the performance of the file traversal by using filepath.WalkDir.
2. A more significant concern with the new test TestUpdateSnippetMetadata, which is currently not testing the replacement logic correctly and modifies files in the testdata directory. I've provided a complete rewrite of the test to make it robust and self-contained, also noting that the suggested fix aligns with the rule of using t.Fatal for error handling in Go tests.

[codecov]

Codecov Report

❌ Patch coverage is 63.15789% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.29%. Comparing base (4e2fdfc) to head (e022cc7).

Files with missing lines	Patch %	Lines
internal/librarian/golang/generate.go	63.15%	4 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3945      +/-   ##
==========================================
- Coverage   83.35%   83.29%   -0.07%     
==========================================
  Files          69       69              
  Lines        6188     6207      +19     
==========================================
+ Hits         5158     5170      +12     
- Misses        671      675       +4     
- Partials      359      362       +3     

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[jameslynnwu]

pull over the doc strings

also, I think we should start logging more things, similar to how legacy librarian does so.