Add host.arch attribute to HostDetector and update tests

[AguirreMoy]

Partially implements #1516

Changes

Adds detection and reporting of the host architecture (host.arch) using System.Runtime.InteropServices.RuntimeInformation.ProcessArchitecture in the HostDetector.

The host.arch attribute is now included in the detected resource attributes on supported platforms. The README has been updated to document the new attribute, and unit tests have been enhanced to verify that host.arch is present and correctly set.

This improves observability by providing architecture information as part of the host resource metadata.

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)
• Unit tests added/updated
• Appropriate CHANGELOG.md files updated for non-trivial changes
• Changes in public API reviewed (if applicable)

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: AguirreMoy / name: Moises Aguirre (188cfd9, 0dd4400, 4431f02, cb65f4c, 7ae7583, ed77e8d, d88c86d, 4e65217, 7c05a1a, 2175618, 3dfcab2, bc8cc66, ea1f32f)
• ✅ login: Kielek / name: Piotr Kiełkowicz (ec7a3f8, 87fefce, 13f45b7, 97cb392, 0142e7b, fdb04ed, 327186e)

[AguirreMoy]

/easycla

[codecov]

Codecov Report

❌ Patch coverage is 61.90476% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.03%. Comparing base (1b1f8b3) to head (97cb392).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/OpenTelemetry.Resources.Host/HostDetector.cs	61.90%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3147      +/-   ##
==========================================
+ Coverage   69.90%   70.03%   +0.12%     
==========================================
  Files         439      429      -10     
  Lines       16919    16888      -31     
==========================================
  Hits        11827    11827              
+ Misses       5092     5061      -31     

Flag	Coverage Δ
unittests-Instrumentation.Cassandra	?
unittests-Resources.Host	71.85% <61.90%> (-2.07%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/OpenTelemetry.Resources.Host/HostDetector.cs	75.20% <61.90%> (-2.90%)	⬇️

... and 10 files with indirect coverage changes

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[Kielek]

Thanks for your contribution. Please fix some issues I have found.

[AguirreMoy]

Hi @Kielek , thanks for the quick review. I've addressed your comments. I marked the easy ones as resolved and left the remainder open for further feedback. Thanks!