Telemetry refactoring

[martincostello]

Cherry-pick changes from #2936:

• Move Meter to TelemetrySource class.
• Use Polly's actual version for the meter's version.
• Extend some test coverage.
• Apply some minor code style refactoring.

[Copilot]

Pull request overview

Introduces a shared telemetry Meter source in Polly.Extensions so telemetry components use a consistent meter name/version derived from the assembly, and adjusts related tests and small builder refactors.

Changes:

• Added TelemetrySource singleton to centralize meter creation/versioning, and updated TelemetryListenerImpl to use it.
• Updated/added tests to validate the new meter behavior and adjusted existing telemetry/builder tests accordingly.
• Minor refactors: reuse ResilienceStrategyTelemetry instance during pipeline build and modernize RegistryPipelineComponentBuilder via a primary constructor.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
test/Polly.Extensions.Tests/Telemetry/TelemetrySourceTests.cs	Adds coverage for the new TelemetrySource meter name/version.
test/Polly.Extensions.Tests/Telemetry/TelemetryListenerImplTests.cs	Updates tests to align with meter version now coming from assembly metadata.
test/Polly.Core.Tests/Telemetry/ResilienceStrategyTelemetryTests.cs	Adds a constructor sanity test and minor cleanup (method group).
test/Polly.Core.Tests/StrategyBuilderContextTests.cs	Makes the test class/method static and improves telemetry assertions formatting.
test/Polly.Core.Tests/ResiliencePipelineBuilderTests.cs	Adds a “copy-ctor reminder” reflection-based guard test.
src/Polly.Extensions/Telemetry/TelemetrySource.cs	New internal telemetry source providing the shared Meter and version parsing.
src/Polly.Extensions/Telemetry/TelemetryListenerImpl.cs	Switches telemetry meter creation to use TelemetrySource.Instance.Meter.
src/Polly.Core/ResiliencePipelineBuilderBase.cs	Reuses a local ResilienceStrategyTelemetry instance during pipeline component building.
src/Polly.Core/Registry/RegistryPipelineComponentBuilder.cs	Refactors to a primary constructor and field initializers.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.15%. Comparing base (af7e8b1) to head (517ce13).
⚠️ Report is 2 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2985   +/-   ##
=======================================
  Coverage   96.15%   96.15%           
=======================================
  Files         309      310    +1     
  Lines        7128     7135    +7     
  Branches     1005     1005           
=======================================
+ Hits         6854     6861    +7     
  Misses        221      221           
  Partials       53       53           

Flag	Coverage Δ
linux	96.15% <100.00%> (+<0.01%)	⬆️
macos	96.15% <100.00%> (+<0.01%)	⬆️
windows	96.15% <100.00%> (+<0.01%)	⬆️

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