test: improve test coverage for pkg/integration/gin

[everfid-ever]

Which issue(s) this PR fixes:

Fixes #950

[Copilot]

Pull Request Overview

This PR adds comprehensive test coverage for the Gin transaction middleware, which handles distributed transaction context propagation in HTTP requests. The middleware extracts transaction IDs (XID) from HTTP headers and initializes the Seata transaction context.

Key changes:

• Adds 8 test cases covering various XID header scenarios (primary key, fallback key, precedence, empty values, missing values)
• Includes integration tests with full Gin router setup
• Validates both success and error paths of the middleware

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 46.38%. Comparing base (4420143) to head (3ffbd0e).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #969      +/-   ##
==========================================
+ Coverage   46.33%   46.38%   +0.04%     
==========================================
  Files         195      196       +1     
  Lines       11887    11901      +14     
==========================================
+ Hits         5508     5520      +12     
- Misses       5943     5944       +1     
- Partials      436      437       +1     

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[Code-Fight]

LGTM

[slievrly]

LGTM