Writing Assistance APIs: Update Writer and Rewriter to new config

Use the new Writing Assistance API session config and options.
Enable rewriter option combinations (WAI, AFAICT) and update tests.
Add enum type conversion and test request checking helper functions.

Overhaul unit tests.

Test: Automated, Writer and Rewriter WAI (ideally better than before)
Bug: 377749376, 380239468, 382596381, 382615217
Change-Id: I8d208e9725942cab7a2066c91bf3a66a7733b7e7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6112741
Reviewed-by: Clark DuVall <cduvall@chromium.org>
Reviewed-by: Brad Triebwasser <btriebw@chromium.org>
Commit-Queue: Clark DuVall <cduvall@chromium.org>
Auto-Submit: Mike Wasserman <msw@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1408164}