[webmcp] Provide parameter descriptions

This CL adds a new attribute (`toolparamdescription`), which can be used
to provide 'description' [1] for individual tool parameters.

As described by Khushal in Issue 22 [2], the value of the 'description'
field is created from multiple sources, as follows:

 1. We prefer an explicit 'toolparamdescription' attribute, if present.
 2. Otherwise, the concatenation of associated label text, if any.
 3. Otherwise, the aria-description attribute.

Note that we currently don't need to update any information when
relevant attributes or other DOM circumstance chances; CL:7428254
introduced code which recomputes the input schema aggressively.

[1] https://json-schema.org/draft/2020-12/json-schema-validation#name-title-and-description
[2] https://github.com/webmachinelearning/webmcp/issues/22#issuecomment-3726418984

Bug: 475972617
Change-Id: I5816e4e54b3162efc6deeaa044a98bccfb3563fb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7510760
Reviewed-by: Dominic Farolino <dom@chromium.org>
Commit-Queue: Anders Hartvoll Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1577100}