Add snapshot configuration API support

[sdairs]

The current live OpenAPI document adds scheduled snapshot configuration after the spec captured by #940. Add typed GET/PATCH support so callers can read the configuration, change its cadence, and disable scheduling while preserving omitted fields.

Fixes #976. Follow-up to #886/#940; the exclusion cleanup remains owned by #908/#913/#914 and their existing PRs.

• Add SnapshotConfiguration, SnapshotConfigurationPatchRequest, both client methods, public exports, beta metadata, and library documentation.
• Vendor the exact live document captured on 2026-09-16 (SHA-256 4c8edc3b2f51f424f7b7dcbb85ae92b79ecd1df4daed2b5df7508c074c5ec7a1), including upstream permission metadata. No analyzer exemptions change.
• Cover request paths/auth/body omission, false/zero values, response tolerance, errors, and all three live lifecycle transitions (enable, cadence-only update, disable). Route backup client/model changes to the service integration suite.

Drift evidence

PR #940's own analyzer reports zero findings against its captured snapshot and ten against this document. The previous stack tip also reports ten; this change reports zero, retaining the same three acknowledged legacy enum constraints. The ten findings are the two operations, two models, two beta entries, and four corresponding snapshot additions.

Validation

• API/analyzer lint and 682 passing tests; all 94 Python tests.
• CLI lint and 1,976 passing tests, telemetry-disabled build/lint, and all-features workspace build.
• Formatting and diff whitespace checks; canonical live-spec dry-run reports zero actionable findings and three legacy acknowledgments.
• Live Cloud integration is authorized for exact head a8299c349c1515ff802e7a3f3dd3b553c8535ae6 and is in progress. Endpoint validation and cleanup are pending; this is not yet live-validated.

Stack

Appended to native GitHub stack #876, directly above #975 (codex/readme-cli-scope-cleanup). The diff contains only this snapshot-configuration follow-up.

[iskakaushik]

LGTM