[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)
• ✅ login: marcalff / name: Marc Alff (473fad3)
• ❌ - login: @ryux1 / name: ryux1. The commit (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 86.43%. Comparing base (e81d957) to head (2d99cec).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4540      +/-   ##
==========================================
+ Coverage   86.41%   86.43%   +0.02%     
==========================================
  Files         524      524              
  Lines       20435    20437       +2     
==========================================
+ Hits        17657    17662       +5     
+ Misses       2778     2775       -3     

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

... and 1 file with indirect coverage changes

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[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