[smartagent] Use system cert pool as default on Windows

[pjanotti]

Description:
The system certificate pool should be the default on Windows, the same behavior of the other supported OSes.

Testing:
Added 2 new tests to cover the new default.

Documentation:
N/A this should be correct default.

[Copilot]

Pull Request Overview

This PR adjusts the default behavior on Windows to use the system certificate pool, aligning Windows with other supported operating systems.

• Added tests to validate the behavior in both HTTP and TLS contexts
• Removed the explicit Windows branch in the TLS configuration code
• Updated the changelog to document the bug fix

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
internal/signalfx-agent/pkg/monitors/http/http_test.go	Added test to verify HTTP monitor metrics when checking Microsoft
internal/signalfx-agent/pkg/core/common/auth/tls_test.go	Added test to ensure CertPool returns the system certificate pool
internal/signalfx-agent/pkg/core/common/auth/tls.go	Updated CertPool to always use the system certificate pool, including on Windows
CHANGELOG.md	Documented change in behavior for Windows certificate pool usage

Comments suppressed due to low confidence (1)

internal/signalfx-agent/pkg/core/common/auth/tls.go:57

• Update the function comment for CertPool to reflect that it now returns the system certificate pool on all platforms, including Windows.

func CertPool() (*x509.CertPool, error) {

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 44.79%. Comparing base (8049f0f) to head (f8dc019).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6240      +/-   ##
==========================================
+ Coverage   44.42%   44.79%   +0.37%     
==========================================
  Files         390      390              
  Lines       26986    26983       -3     
==========================================
+ Hits        11988    12088     +100     
+ Misses      14156    14022     -134     
- Partials      842      873      +31     

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