[wip] test(auth): migrate log/statsd/fxaMailer mocks to typed Jest fixtures

[nshirley]

Because:

• test/mocks.js is an untyped ~1.2k-line monolith whose shared factories obscure test intent and cannot track production-type drift (FXA-13708).

This commit:

• Replaces mocks.mockLog()/mockStatsd() with inline createMock() and createMock() (@golevelup/ts-jest) across ~60 specs, plus any hand-rolled statsd stubs.
• Adds test/fixtures/ (README + fxa-mailer.ts with installMockFxaMailer/ uninstallMockFxaMailer) and migrates the 16 route specs off mocks.mockFxaMailer(), with Container teardown.
• Completes the AuthLogger interface (flowEvent, activityEvent, amplitudeEvent, summary) so the typed log mock matches the real logger.
• Removes the now-dead mockStatsd and mockFxaMailer slices from test/mocks.js (mockLog stays; mockRequest still consumes it internally).
• Adds @golevelup/ts-jest as a devDependency. Legacy mocks.mockMailer() and its negative "not called" guards are left intact; no file tests the legacy mailer sending path.

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[nshirley]

I'll remove the comment here, but left it for the time being to remind myself.

I suspect, as more things are ripped out of the shared mocks.ts, we'll see this more. There was a hidden circular dependency. The import here was a re-export from the source, but the mocks.ts also imported all of the relevant modules in order, thus loading them into the dependency cache and hiding this problem.