docs(saas-api): document 25MB max value size and 413 response

[27Bslash6]

Documents the maximum cache value size and the 413 response in spec/saas-api.md:

• PUT /v1/cache/{key}: max value size 25 MB → 413 Payload Too Large (permanent; SDKs MUST NOT retry, surface "value too large"). The ceiling MAY change, so SDKs treat any 413 as value-too-large. Unrelated to the SDK serializer's 512 MB in-memory bound.
• Adds 413 to the HTTP status-code table and the Permanent error class.

Docs-only; client-facing API contract.

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• spec/saas-api.md

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Walkthrough

The SaaS API spec adds a 25 MB maximum cache value size constraint, with 413 Payload Too Large defined as a permanent, non-retryable error. The 413 status is added to the cache PUT status table, the general HTTP error handling table, and the permanent-error classification table.

Changes

413 Payload Too Large — 25 MB value size limit

Layer / File(s)	Summary
413 permanent error and 25 MB value size constraint spec/saas-api.md	Adds the 25 MB maximum stored value size rule with 413 Payload Too Large as a permanent non-retryable error in the protocol constraint block and cache PUT status table; adds 413 to the general HTTP status table and the permanent-error classification row.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The PR title clearly and specifically summarises the main change: documenting a 25MB maximum value size limit and the corresponding 413 response for the SaaS API.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch docs/saas-api-413-max-value

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