agentHost: add Claude and Codex session telemetry (#333274)

* agentHost: add Claude and Codex session telemetry

Generalize Agent Host subscription telemetry across providers while preserving Copilot-specific SDK resume dimensions. Add coverage for Claude, Codex, and future providers.\n\n(Written by Copilot)\n\nCo-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* agentHost: filter session telemetry by registered provider

Resolve subscription providers through the provider service so future registered providers are included while terminal and unknown resource schemes are excluded.\n\n(Written by Copilot)\n\nCo-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* agentHost: use canonical provider service interface

Keep the production dependency typed as IAgentHostProviderService and move partial service stubbing into test code.\n\n(Written by Copilot)\n\nCo-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* docs: require canonical injected service interfaces

Document that production service dependencies use their canonical interfaces and keep partial test conveniences in test code.\n\n(Written by Copilot)\n\nCo-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>