[CODE HEALTH] Fix clang-tidy narrowing conversions in baggage

[thc1006]

Fixes #3980
Contributes to #2053

Changes

• Wrap three implicit int -> char conversions inside Baggage::UrlEncode
  and Baggage::UrlDecode with explicit static_cast<char>(...) so that
  cppcoreguidelines-narrowing-conversions stops firing under clang-tidy 20.
• Decrement the clang-tidy.yaml warning_limit by 3 for both
  all-options-abiv1-preview and all-options-abiv2-preview matrix entries
  to ratchet the limit down to the new measured count.

The arithmetic is unchanged: c >> 4 and c & 15 are already bounded to
[0, 15] by the existing shift/mask, and the bitwise OR of two from_hex
nibbles in UrlDecode fits in a single byte. PR #1353 already added the same
static_cast<char> inside the from_hex lambda body back in 2022; this
change just finishes the job at the call sites that clang-tidy 20 newly
flags.

Fixes the following warnings:

---

opentelemetry-cpp/api/include/opentelemetry/baggage/baggage.h (3 warnings)

Line	Check	Message
238	cppcoreguidelines-narrowing-conversions	narrowing conversion from 'int' to signed type 'char' is implementation-defined
239	cppcoreguidelines-narrowing-conversions	narrowing conversion from 'int' to signed type 'char' is implementation-defined
270	cppcoreguidelines-narrowing-conversions	narrowing conversion from 'int' to signed type 'char' is implementation-defined

---

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[Copilot]

Pull request overview

Addresses new cppcoreguidelines-narrowing-conversions clang-tidy warnings in the Baggage URL encode/decode helpers and adjusts the clang-tidy CI threshold accordingly, keeping the project’s warning budget ratcheting downward.

Changes:

• Add explicit static_cast<char>(...) at three previously-implicit int -> char conversion sites in Baggage::UrlEncode / Baggage::UrlDecode.
• Lower warning_limit in the clang-tidy GitHub Actions workflow for the two preview matrix entries to match the reduced warning count.
• Add an Unreleased changelog entry for the code health change.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
api/include/opentelemetry/baggage/baggage.h	Makes narrowing conversions explicit to satisfy clang-tidy’s narrowing-conversions check.
.github/workflows/clang-tidy.yaml	Updates warning thresholds to reflect the reduced clang-tidy warning count.
CHANGELOG.md	Records the code health change in the Unreleased section.

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.17%. Comparing base (185dbd7) to head (31c4c4a).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3989      +/-   ##
==========================================
- Coverage   90.18%   90.17%   -0.01%     
==========================================
  Files         230      230              
  Lines        7299     7299              
==========================================
- Hits         6582     6581       -1     
- Misses        717      718       +1     

Files with missing lines	Coverage Δ
api/include/opentelemetry/baggage/baggage.h	98.28% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[dbarker]

Looks good. Thanks for the contribution!

[marcalff]

LGTM, thanks for the fix.