Make terminal-host telemetry opt-in (#20005)

* Make terminal-host telemetry opt-in

Disable helper telemetry unless ShowTerminalHost is enabled, and preserve per-helper OTLP service identity for diagnostic runs.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 1c676f42-11fe-4ca3-9dba-8beeeca95fca

* Fail closed for malformed terminal telemetry opt-in

Use bool.TryParse so invalid diagnostic configuration leaves telemetry disabled without terminating the terminal host. Cover empty, whitespace, malformed, and numeric values.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 1c676f42-11fe-4ca3-9dba-8beeeca95fca

* Use shared configuration parsing for terminal telemetry

Link IConfigurationExtensions into TerminalHost using the existing non-hosting attribute shim pattern, and use GetBool with a false default. Cover zero and nonzero numeric opt-in values alongside malformed input.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 1c676f42-11fe-4ca3-9dba-8beeeca95fca

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Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 1c676f42-11fe-4ca3-9dba-8beeeca95fca