Keep blank and null optional arguments compatible with MCP hosts that cannot
omit advertised properties. Empty Mail sender, mailbox, thread and message
filters retain their defaults, and null defer_until remains omitted.

Preserve explicit empty message content, Mail text and subjects so a draft
revision cannot silently send its previous body. Keep unknown properties
visible to input-schema validation. Add matching Rust and TypeScript
regression tests and document the normalization boundary in both SDK READMEs.

Align all three SDK versions and both landing-page languages at 0.12.5,
and record the compatibility fixes and corrected mail_list read state in
the changelog.

Validated all three SDK suites, Mail interoperability, TypeScript build,
Rust format and clippy, default-feature Rust tests, catalog parity, and
bilingual browser demos.

Co-authored-by: Codex <noreply@openai.com>