feat(mri): expose skip initial run on leaderboard creation

[RapidPoseidon]

Exposes the backend's skipInitialRun flag on benchmark.create_leaderboard, added in rapidata-backend#5141.

leaderboard = benchmark.create_leaderboard(
    name="Staged Evaluation",
    instruction="Which image do you prefer?",
    skip_initial_run=True,
)

Defaults to False, so every existing caller keeps queueing the initial run exactly as before. The parameter is appended last, so positional callers are unaffected too.

The leaderboard then starts with no responses and therefore no standings. Everything else keeps working: a model added afterwards is compared against the whole existing field, and the leaderboard can still be boosted from the app.

Notes

• The generated client already had the field — CreateLeaderboardEndpointInput.skip_initial_run arrived with the automated regeneration in Update OpenAPI Schemas #831 (pinned to the backend merge commit f4aff59). This PR is purely the high-level wrapper.
• Nothing is stored on RapidataLeaderboard. The backend treats it as a creation-time decision and does not record it, so there is no property to read back — same as included_tags being create-only, except that one at least echoes locally.
• No boost method to point at. The docstring says "boosting the leaderboard still works" rather than referencing a method, because boosting is not exposed in the SDK.

Tests

tests/rapidata_client/benchmark/test_leaderboard_skip_initial_run.py — the flag reaches the wire when set, and is explicitly False (not omitted) by default, so the request never leans on the server-side default to preserve today's behaviour.

Also ran: pyright src/rapidata/rapidata_client clean, pytest tests/rapidata_client/benchmark 65/65.

Four tests in tests/rapidata_client/audience/ fail on this branch, but they fail identically on a clean main — pre-existing and unrelated (test_assign_job_*).

🔗 Session: https://poseidon.rapidata.internal/chat/session-66b1bde9

[LinoGiger]

not something we wanna encourage so just the code doc should be fine instead of the main mri docs

[RapidPoseidon]

Done in 6991e07 — dropped the section from docs/mri_advanced.md, so the flag is documented only in the create_leaderboard docstring (and therefore the API reference). The diff against main is now just the wrapper parameter and its tests.