feat(context): add generic Value and ValueOr methods for Go 1.27

[EricGusmao]

Summary

• Add Go 1.27-gated generic Context.Value and Context.ValueOr
  methods.
• Refactor context lookups into shared helpers.
• Preserve missing-key and invalid-type error behavior.
• Add tests covering successful lookups, defaults, missing keys,
  invalid casts, and nil values.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.71%. Comparing base (dcb05f0) to head (3d4a795).
⚠️ Report is 11 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3075      +/-   ##
==========================================
+ Coverage   93.34%   95.71%   +2.36%     
==========================================
  Files          43       45       +2     
  Lines        4735     4039     -696     
==========================================
- Hits         4420     3866     -554     
+ Misses        192      173      -19     
+ Partials      123        0     -123     

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