s/Test Analytics/Test Engine/g

[pda]

Update README and comments to refer to Test Engine, not Test Analytics.

[Copilot]

Pull Request Overview

This PR updates the documentation and comments to use the new "Test Engine" branding instead of the deprecated "Test Analytics" terminology. This appears to be a rebranding effort to align with Buildkite's updated product naming.

• Update module docstrings in Python files to reference "Test Engine" instead of "Test Analytics"
• Update README.md to use "Test Engine" terminology throughout documentation
• Update links in README.md to point to the new Test Engine documentation URLs

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/buildkite_test_collector/collector/payload.py	Updated module docstring to reference "Test Engine"
src/buildkite_test_collector/collector/api.py	Updated module and class docstrings to reference "Test Engine"
README.md	Updated all documentation text and links to use "Test Engine" branding