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]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.48%. Comparing base (30322cf) to head (d86d10a).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4540      +/-   ##
==========================================
+ Coverage   83.47%   83.48%   +0.01%     
==========================================
  Files         522      522              
  Lines       20440    20442       +2     
==========================================
+ Hits        17061    17064       +3     
+ Misses       3379     3378       -1     

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

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[owent]

LGTM after CLA is signed.