fix(ai)!: keep session settings on MCP session fetches (#418)

The three MCP tools that accept a `session_id` forwarded every per-fetch
parameter to the session's `fetch()`, including their own defaults when
the caller supplied nothing. `validate_fetch` treats every key it
receives as an override, so the branch that reads the value from the
session config never ran, and the tool defaults replaced the settings the
session was opened with.

- Forward only the parameters the caller actually supplied
- Default those parameters to `None` on `fetch`, `bulk_fetch`,
  `stealthy_fetch`, `bulk_stealthy_fetch` and `screenshot`
- Add regression tests for the forwarded kwargs, and for the session
  settings surviving the validation the tools go through