fix(session): use finite archived timestamp schema

[kitlangton]

Summary

• Use Schema.Finite for session archive timestamps so generated OpenAPI/SDK types stay number without non-finite string literal arms.
• Keep negative archive timestamp values accepted for legacy compatibility while rejecting values that cannot round-trip through JSON.
• Add bridge coverage for the SDK-affecting session update request schema.

Validation

• bun typecheck
• bun run test test/server/httpapi-bridge.test.ts test/server/httpapi-mcp-oauth.test.ts
• pre-push bun turbo typecheck