docs(auth): correct the auth README against the GA API surface, and fix two broken exception messages

[demolaf]

auth/README.md documented an API that does not exist: AuthState as a sealed hierarchy of data classes, an MFA section that builds a config object and never passes it anywhere, and an error when covering half the exception types with no else. Corrected against the source on pre-GA, with the hand-maintained test-tag table replaced by the naming rule the test suite already enforces.

Changes

• AuthState block: abstract not sealed, nothing is a data class, Error.exception is typed Exception, plus Success.reauthenticatedUid, isNotification and the AuthState.Reauthentication hierarchy, with a note on what each means for caller code.

• MFA Configuration now passes mfaConfiguration to FirebaseAuthScreen, which is where it goes. It is a screen parameter, not a property of authUIConfiguration, and the old example dropped the object it built.

• Error Handling covers all 21 AuthException subclasses and ends with an else, since AuthException is not sealed and the missing branches previously failed silently.

• Test tags: the 32-row table becomes the fui_<object_snake_case>_<CONSTANT_lowercase> rule, the 13 grouping objects, and a pointer to FirebaseAuthTestTags, citing the reflective assertions in FirebaseAuthTestTagsTest.

• The stability promise is rescoped from "a value documented here" to every constant on FirebaseAuthTestTags, bringing the 27 constants the table omitted inside it.

• Sign Out documents that signOut clears Google Credential Manager state and the Facebook LoginManager session for linked providers, with the Facebook step guarded by a classpath probe.

• The FirebaseAuthScreen parameter table is marked partial and points at Custom UI with Slots for the seven slot parameters; the drifting "~170 abstract vals" count is dropped rather than re-pinned.

• Fixes the localization example, which defined SpanishStringProvider with no constructor parameters and then called SpanishStringProvider(context), so the snippet did not compile.

• Repairs two AuthException messages that read "You are are attempting...", the second also missing a word. Own commit, since it is source rather than docs.

Validation

Five sweeps re-run on the finished README: symbol resolution, named arguments against each callee, required arguments against each signature, Compose calling context, and anchor/TOC resolution (56 anchors, all resolve, modelling GitHub's duplicate-heading suffixing).

The string change carries ./gradlew :auth:testDebugUnitTest --rerun-tasks: BUILD SUCCESSFUL, 49 tasks executed, 1101 tests, 0 failures or errors. Verified as a real run rather than cached results by diffing the 82 test classes under auth/src/test against the 82 in auth/build/test-results, none missing, all XMLs written during this run.

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Maintainer note: Refs internal CPRN-490

[gemini-code-assist]

Code Review

This pull request updates the auth/README.md documentation to reflect API changes in AuthState (transitioning from a sealed class to an abstract class), and adds detailed guidance on MFA configuration, test tags, sign-out behavior, and error handling. The feedback points out that the companion object properties in the documented AuthState class must be initialized to be valid, compilable Kotlin, and provides a code suggestion to fix this.

[gemini-code-assist]

In Kotlin, properties declared inside a companion object must be initialized. To ensure the documented code block is valid and compilable Kotlin, please initialize these companion properties to match their actual implementation in AuthState.kt.

Suggested change

	@JvmStatic val Idle: Idle
	@JvmStatic val Cancelled: Cancelled
	@JvmStatic val Aborted: Aborted
	@JvmStatic val Idle: Idle = Idle()
	@JvmStatic val Cancelled: Cancelled = Cancelled()
	@JvmStatic val Aborted: Aborted = Aborted()