fix(pull_request_read): reject pagination parameters the selected method does not support

[larik15]

What

pull_request_read accepts two pagination styles in one schema — page/perPage
for the REST-backed list methods and after (a GraphQL cursor) for
get_review_comments. A parameter the selected method could not use was
dropped silently, so a caller passing after to get_files received the
first page again with no signal that nothing had advanced.

This adds a guard before the method dispatch that returns a tool error naming
the method and the unsupported parameter:

• get_review_comments rejects page
• get_files, get_commits, get_reviews, get_comments, get_check_runs reject after
• get, get_diff, get_status reject all three, since they return a single object

Calls that already use the right parameters are unaffected.

Why

For an LLM caller a schema description is guidance, not enforcement. The
description for after already says it is only for get_review_comments,
but the model still sees a pagination cursor in the schema and reaches for
it when it wants the next page of files. Silent drops are the worst outcome
for that caller: the response is plausible, so it never retries.

Follow-up to #2489, which exposed after in the schema but did not add the
guard; same failure class as #2347.

Testing

go test ./pkg/github/... passes. Added Test_rejectUnsupportedPagination
covering the accepted and rejected combination for each method group.

Fixes #3316