Fix regression-runner scan timeout ownership

[DavidBakerEffendi]

Keep Bifrost scan deadline policy in UsageBench without restoring the removed max_duration_secs tool argument. The runner now scopes the configured budget to the scan call, cancels timed-out MCP work, and leaves subsequent requests usable.

Key Changes:

• Replace the session-wide timeout mutation with a per-call deadline covering readiness retries and backoff.
• Send standard MCP notifications/cancelled on transport timeout and preserve explicit runner-error reporting.
• Add regressions for omitted server arguments, timeout error mapping, cancellation payloads, stale responses, next-call recovery, zero duration, and logical deadline bounds.

Touch Points:

• src/runners/mcp.rs
• src/runners/bifrost.rs