fix(schema): require integer retry_after seconds

[bokelley]

Summary

Makes error.retry_after integer-only for 3.2 and pins migration behavior rather than leaving fractional values to SDK-specific truncation.

The rule is ceiling before clamping:

1. Round a finite fractional delay up to the next whole second.
2. Clamp the result to 1–3600.
3. Treat non-finite values as absent.

Rounding up preserves the producer's minimum intended delay; flooring could make a client retry early. During migration, tolerant clients should apply the same rule to legacy fractional values, while 3.2 producers emit integers only.

Closes #5953.

Changes

• Changes the JSON Schema type from number to integer.
• Aligns error vocabulary and both error-handling references.
• Updates the executable JavaScript example to use Math.ceil before clamping.
• Adds a regression test that pins type, bounds, and migration prose.
• Adds a minor protocol changeset.

Validation

• Retry-after regression tests: 2 passed
• Error-code and drift lints: clean
• Error-handling copy test: passed
• Changeset scope gate: passed
• git diff --check

[aao-secretariat]

MUST FIX: number → integer narrows the value domain of a field released in 3.1.x. Every payload valid under the old schema (retry_after: 30.5) is now schema-invalid. That is a backward-incompatible tightening of a published wire contract, not an additive change — a 3.2 consumer that validates a 3.1 seller's error strictly against this schema rejects it. The PR's own migration prose ("legacy finite fractional value") confirms fractional values exist on the wire, so this is not a clarification every conformant implementation already satisfies (the prior reference example did not ceil). Constitution decision class: Breaking → major, not the minor changeset shipped. Repo rule: a minor changeset carrying a breaking wire change is high. Either resize to major (and route through Breaking-class ratification) or state in the PR body why a domain narrowing on a released field is being treated as non-breaking.

[aao-secretariat]

Ladon verdict: Request changes

Request changes — 1 blocking finding.

Blocking findings

• static/schemas/source/core/error.json:27 — retry_after type narrowed from number to integer, a backward-incompatible wire tightening shipped on a minor changeset. A field released in 3.1.x is having its value domain narrowed; a producer that previously emitted a fractional retry_after now emits an invalid value against the tightened schema, breaking conformant 3.1 implementations. Per the changeset-scope rule, a breaking wire change (type narrowing / value-domain restriction on a published field) requires a major changeset, not a minor. This is also a Breaking-class protocol change under the constitution's decision classes, which is never auto-approved without human ratification.

The schema↔docs coherence, ceil-before-clamp migration prose, JS example, and regression test are all internally consistent and correct — the sole concern is the release sizing of the breaking narrowing.

Note: this PR also touches gated paths (static/schemas/source/**) with review_decision: REVIEW_REQUIRED, so it additionally requires human/CODEOWNERS approval before merge.

Row 1 of the decision table (a high finding present) governs: outcome is request-changes.

Blocking findings

• static/schemas/source/core/error.json:27 — retry_after number -> integer is a breaking type narrowing shipped as minor; breaking wire changes require a major changeset and human ratification