Detect concealed characters in server metadata

[Adeniyikayodee]

Closes #1121

Summary

Adds a check on the client listing paths for characters in server metadata
that stay invisible to a reviewer while still reaching the model.

Motivation

A server advertises a tool through tools/list with a name, a description
and a schema. A host typically renders this once for approval, then supplies
the same bytes to the model on every later turn. MCP allows the rendered view
and the delivered bytes to differ, so a code point that lacks a glyph is
absent from what a person approves and arrives at the model intact.

Approach

Two classes of character are reported:

• The Unicode TAG block, U+E0000 to U+E007F. Mainstream terminals, chat
  clients and IDEs leave it unrendered, and it lacks a legitimate use in tool
  metadata.
• Bidirectional overrides and isolates, U+202A to U+202E and U+2066 to
  U+2069, which reorder how the surrounding text is displayed.

Zero width joiners and the remaining format characters are left alone, since
they carry meaning in Indic, Arabic and Persian text and in emoji sequences.
A test covers שלום, مرحبا, नमस्ते and a joined family emoji.

Metadata is reported and left as it arrived. A stripped description is still
text the server chose, and rewriting a name would break the calls that use
it.

The shape follows ToolNameValidator, which already runs on the same listing
path: a public final class in util, static methods, one system property.

Configuration

Warn by default, so current behaviour is preserved. Set
io.modelcontextprotocol.strictMetadataValidation=true to fail the listing
instead.

Tests

11 unit tests and 3 client level tests. A payload is encoded into the TAG
block and confirmed in a description, inside a nested JSON schema, and in map
keys. The client tests cover the default path and the strict path.

Verification

./mvnw clean test passes across all 11 modules, 1496 tests, conformance
included.

Open question

Whether the character set stays fixed as written or becomes configurable.
Fixed seems safer to me... a configurable blocklist invites someone to add
the whole Cf category, which would fire on Persian and Indic descriptions.