Rust: expose prompt-event observer configuration

[examon]

[examon's robot driving]

Expose optional observe_prompt_events configuration and builders on Rust session creation and resume, serialized as observePromptEvents.
The field is omitted by default and does not change callback-registration flags.
Event-driven hosts can use request IDs to answer user_input.requested and retire their UI on user_input.completed, including when another client answers.
The API documentation makes the observer's responsibility for pending questions explicit.

The wire regression fails when the configured value is dropped while the default-shape control still passes.
With the implementation restored, all 231 Rust unit tests pass, along with the repository's nightly formatting, configured lint checks and all-feature documentation build.
Supplemental no-default-features lint reproduces two unused test-helper diagnostics on unchanged main; the repository's CI lint configuration passes.