gapic-generator-typescript emits ESLint-violating output (promise/*, prefer-const)

[bshaffer]

Split out from #9429, which works around this with an .eslintrc.json override.

Problem

Four generator templates emit code that violates the repo's root ESLint config, so every librarian regeneration PR fails bin/linter.mjs --strict (#9427 had 59 errors).

Rule	Template	Issue
promise/always-return	templates/{cjs,esm}/typescript_gapic/src/$version/$service_client.ts.njk (close())	return this.xStub.then(stub => { ... }) — callback returns nothing
promise/always-return	templates/{cjs,esm}/typescript_gapic/test/gapic_$service_$version.ts.njk (both close() tests)	client.close().then(() => { done(); })
@typescript-eslint/no-floating-promises	same test template, Locations/IAM mixin callback tests	client.getLocation(req, opts, cb) — the callback overload still types as returning Promise
prefer-const	templates/{cjs,esm}/typescript_gapic/_util.njk:268	let routingParameter = {} is only ever Object.assign'd — plain bug

Latent debt

bin/linter.mjs is diff-scoped (eslint.lintFiles() on changed files only), so checked-in generated code already carries violations CI never sees. Untouched files on main today:

• packages/google-cloud-run/src/v2/jobs_client.ts → 9 errors (8x prefer-const, 1x promise/always-return)
• packages/google-storage-control/src/v2/storage_client.ts → 37 errors (9x prefer-const, 1x promise/always-return, 20x prettier/prettier)

16 checked-in files contain let routingParameter.

The prettier/prettier count suggests the templates have formatting drift beyond these four rules; worth a full audit.

Removing the workaround

Fixing the templates on main is not sufficient. librarian.yaml pins the generator to a repo tag fetched as a checksummed tarball:

- name: gapic-generator-typescript
  version: gapic-generator-v5.5.0
  package: https://github.com/googleapis/google-cloud-node/archive/gapic-generator-v5.5.0.tar.gz
  checksum: dec0756bf04ff1cb881cd7a8a51c538fd5d25daedd1b65d93800632d85baddea

So the sequence is:

1. Fix the four templates (cjs + esm) and update generator baselines
2. Cut a new gapic-generator-v* tag
3. Bump version + checksum in librarian.yaml
4. Drop the @typescript-eslint/no-floating-promises and prefer-const lines from the generated-GAPIC override in .eslintrc.json

Note the two promise/* lines are a restoration of pre-#8458 behavior (the promise plugin was added to the root config in #8446, three weeks after per-package root: true configs were deleted), so those can stay off permanently or be dropped in the same step — either is defensible.