fix(agent): neutralize : in cancellation registry keys to prevent delimiter collision

[chuenchen309]

Problem

cancel_key(project_id, session_id, run_id) joins the three fields with a
literal "::" delimiter, but normalize_key() only neutralized path
separators (\ and /), not ::

fn normalize_key(value: &str) -> String {
    value.replace(['\\', '/'], "_")
}

session_id / run_id are unrestricted Option<String> fields (no charset
validation, unlike agent/session.rs's sanitize_session_id) and are
reachable from the local HTTP API's POST /projects/:id/chat body and the
agent_chat / agent_start_turn_stream Tauri commands. When a field contains
the substring "::", two distinct (session_id, run_id) pairs collapse onto
the same registry key — e.g. (session_id="a::b", run_id="c") and
(session_id="a", run_id="b::c") both key to "p1::a::b::c" — so cancelling
one turn cross-cancels an unrelated turn's cancellation token. The no-run_id
prefix-search path in cancel() has the same exposure.

Fix

Extend normalize_key to also replace : → _, matching its existing
\ / / → _ pattern, so the :: delimiter can no longer appear inside a
field value.

Test

Added cancellation_registry_does_not_collide_on_embedded_delimiter to the
existing #[cfg(test)] mod tests, asserting the two colliding pairs above stay
independent.