[SDK] Avoid throwing for non-string process executable names

[ryux1]

Fixes #4535

Changes

• read process.executable.name with nostd::get_if when deriving the fallback service name
• fall back to unknown_service when the attribute exists with a non-string value
• preserve unknown_service:<process.executable.name> for valid string values
• add regression coverage for both paths

Validation

• reproduced the original bad_variant_access failure with the new regression test before applying the fix
• all 16 resource tests pass with C++14 under ABI v1
• all 16 resource tests pass with C++14 under ABI v2
• opentelemetry_resources builds with C++14 and -fno-exceptions
• clang-format 18 dry-run passes

For significant contributions:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed (this PR does not change the public API)

Implementation, tests, and this description were prepared with OpenAI Codex assistance.

[linux-foundation-easycla]

• ✅ login: ThomsonTan / name: Tom Tan (2d99cec, 7fbbe45)
• ✅ login: marcalff / name: Marc Alff (473fad3, f9e24f8)
• ❌ - login: @ryux1 / name: ryux1. The commit (4a09f0e, e88afb7) is not authorized under a signed CLA. Please click here to be authorized. For further assistance with EasyCLA, please visit our EasyCLA portal and chat with our support bot.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.51%. Comparing base (044b114) to head (4a09f0e).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4540      +/-   ##
==========================================
+ Coverage   86.51%   86.51%   +0.01%     
==========================================
  Files         525      525              
  Lines       20464    20466       +2     
==========================================
+ Hits        17702    17705       +3     
+ Misses       2762     2761       -1     

Files with missing lines	Coverage Δ
sdk/src/resource/resource.cc	97.23% <100.00%> (+3.11%)	⬆️

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[owent]

LGTM after CLA is signed.

[marcalff]

/easycla

[marcalff]

LGTM, thanks for the fix.

[marcalff]

@ryux1

Thanks for the fix.

Please sign the EasyCLA, this is needed to merge.

[marcalff]

/easycla

[ryux1]

Merged current main in 4a09f0e and resolved the PR conflict. The only conflict was in CHANGELOG.md; both the resource fix entry and the upstream entry are preserved. Upstream had not changed the resource implementation or its tests since this PR's base, and the merged tree passes git diff checks. The PR is mergeable again; EasyCLA remains the external gate.