Generate a case-count report on pull requests

[kainino0x]

And update the parameterization docs and checklist.

This adds about 2 minutes to the CI run (one minute each to generate the report before and after the PR).

This also makes it easy for us to add additional text to the bot comment if we want to later. (We might do that if we re-enable previews, which were disabled in #3131.)

Here's a sample comment from kainino0x#1

Results for build job (at 28ceb80):

-webgpu:examples:basic,builder_cases_subcases:* - 2 cases, 4 subcases (~2/case)
+webgpu:examples:basic,builder_cases_subcases:* - 3 cases, 9 subcases (~3/case)
-TOTAL: 284493 cases, 2341915 subcases
+TOTAL: 284494 cases, 2341920 subcases

Issue: None

---

Requirements for PR author:

• All missing test coverage is tracked with "TODO" or .unimplemented().
• New helpers are /** documented */ and new helper files are found in helper_index.txt.
• [n/a] Test behaves as expected in a WebGPU implementation. (If not passing, explain above.)
• [n/a] Test have be tested with compatibility mode validation enabled and behave as expected. (If not passing, explain above.)

Requirements for reviewer sign-off:

• [n/a] Tests are properly located in the test tree.
• [n/a] Test descriptions allow a reader to "read only the test plans and evaluate coverage completeness", and accurately reflect the test code.
• [n/a] Tests provide complete coverage (including validation control cases). Missing coverage MUST be covered by TODOs.
• Helpers and types promote readability and maintainability.

When landing this PR, be sure to make any necessary issue status updates.

[kainino0x]

Test failure is expected because it tries to run the previous version of tools/validate with the new flag. Little janky but I think it'll be fine, I'll force merge.

[kainino0x]

Sigh, not working because the test PR is from a fork (https://github.com/orgs/community/discussions/25220). Will leave it on while I try to figure it out, it doesn't block PRs.

[kainino0x]

Took a few revisions: 4ed1a40...8f6f731
but seems to work now: #4549