Preserve language and direction for declarative tool descriptions

[mlmrx]

Summary

The declarative API explainer places natural-language tool metadata in the
tooldescription and toolparamdescription HTML attributes, then turns those values into a tool
description and JSON Schema property descriptions. The current proposal does not define how the
language or base direction of those strings is determined or preserved during that conversion.

This is the declarative-markup part of #153, identified while reviewing the checklist in #273.

Current text

• tooldescription is analogous to ModelContextTool.description.
• toolparamdescription becomes a JSON Schema description.
• The normative schema-synthesis algorithm is still a TODO, so this can still be resolved before the markup is fixed in place.

Internationalization concern

W3C's Internationalization Best Practices for Spec Developers says that:

• the language and string direction must be determinable for every natural-language string field;
• natural-language text in attribute values should be avoided;
• if user-readable text is defined in attributes, language and direction information must be available separately.

The current explainer does not say:

1. whether these new attributes are translatable;
2. whether their values use the form control's inherited HTML lang and dir/auto context; or
3. how that context is carried into the synthesized tool definition and JSON Schema instead of
   being reduced to an unlabelled string.

This matters even when a description is primarily agent-facing: property descriptions and other
schema annotations may be surfaced in confirmation or native UI, and the tool title is explicitly
defined for user interfaces.

Resolution criteria

Before defining the declarative synthesis algorithm in #210, decide whether to:

• represent descriptions as element content (allowing ordinary HTML lang, dir, and inline
  markup), or
• keep attributes but normatively define their translation, language, and direction behavior and
  preserve that metadata in the synthesized representation.

The solution should also state how authors provide localized alternatives, or explicitly defer that
part to the resolution of #153.

Maintainers: please apply the i18n-tracker label so the Internationalization WG is notified, as
requested by #273. I cannot apply repository labels with my current permissions.