[LF3551]

Fixes #360.

Adds a mitigation under Security and Privacy Considerations → Mitigations for invisible characters (Unicode tag characters, zero-width characters, bidi controls) in tool titles, descriptions, and schemas. Non-normative, as requested in the issue.

It covers why these characters get past review, approval UIs, and keyword or small-model guardrails, why tool names and length limits don't address it, and the practical points from @seunghan91's comment:

• match on code points rather than UTF-16 code units, since tag characters are surrogate pairs;
• replace lone surrogates before removing anything (description is a DOMString, unlike the USVString title; in the serialized schema they appear as \uXXXX escapes, so this applies to the parsed schema);
• use Default_Ignorable_Code_Point as a baseline instead of a hand-written list;
• don't strip values the tool receives (enum, const, default, property names);
• show the user the same text the model gets.

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