Expand v3 changelog from repository history

[Copilot]

The 3.0.0 changelog entry only captured the initial runtime retargeting and missed the broader set of v3 changes. This updates CHANGELOG.md to reflect the full v3 scope based on the repository history.

• Release notes coverage

  • Expands the 3.0.0 entry beyond the existing .NET 10 note.
  • Captures the major API additions introduced during the v3 line, including cancellation token support, paged results, DI registration, and HTTP diagnostics.

• Platform and architecture changes

  • Summarizes the major breaking/runtime shifts in v3: .NET 10 retargeting, centralized project configuration, nullable enablement, and the move to System.Text.Json.
  • Includes API surface cleanup such as async naming alignment and HttpClient ownership updates.

• Docs, workflow, and reliability updates

  • Adds the notable documentation, CI, release-process, and integration-environment work that shipped as part of v3.
  • Records key fixes that were otherwise absent from the changelog, such as comment serialization/meta handling and improved self-hosted test environment stability.

• Example

  ## [3.0.0]
  
  ### Added
  
  - `CancellationToken` support across the public async API surface.
  - `PagedResult<T>` plus paged query/get helpers for exposing pagination metadata.
  
  ### Changed
  
  - Retargeted the library, tests, and CI/tooling to .NET 10.
  - Migrated serialization from `Newtonsoft.Json` to `System.Text.Json`.

[Copilot]

Pull request overview

Expands the 3.0.0 changelog entry to reflect the broader v3 feature set and breaking/runtime changes based on repository history.

Changes:

• Added a fuller Added/Changed/Fixed breakdown for 3.0.0 (API additions, diagnostics, docs).
• Documented v3 platform/architecture shifts (nullable enablement, System.Text.Json, centralized build config, HttpClient ownership).
• Recorded notable reliability/docs fixes that were missing from the original entry.

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