[transports-http/diagnostics] Validate diagnostics response-size limits before endpoint registration

[kang-heewon]

Priority: P1

Problem

Diagnostics endpoint limits are copied without validation. Negative, non-finite, or fractional values reach slice and message truncation, making bounded output unreliable.

Evidence

• packages/transports-http/src/libs/operationalEndpoints.ts:49-63 accepts recentErrorLimit and messageLimit.
• Lines 121-140 copy the values into policy without validation.
• Lines 184 and 233-243 consume them in slice and message capping.

Desired outcome

Operational diagnostics have explicit, safe output bounds.

Implementation path

1. Validate a finite nonnegative error count and finite positive message limit.
2. Return a stable configuration Problem before exposing the endpoint.
3. Test negative, NaN, Infinity, fractional, and normal options.

Acceptance criteria

• Invalid limits are rejected before requests are served.
• Valid limits cap error entries and messages predictably.
• Error messaging identifies the invalid option.

Validation

pnpm --filter @croco/transports-http test

Scope boundaries

Does not change timeout configuration covered by existing health work.