port: MFA getAuthenticators type-filtering + TOTP field mapping to v4

[pmathew92]

Summary

Ports PR #998 (two related MFA fixes, merged to main) into v4_development, following the feature-by-feature port model. Same 4-file scope as #998; hunks hand-applied onto v4's structure.

1. getAuthenticators — filter on Authenticator.type (exact match)

• MfaApiClient.getAuthenticators(factorsAllowed) now filters by comparing each Authenticator.type directly against factorsAllowed (exact, case-sensitive), replacing the previous oob_channel/authenticator_type "effective type" derivation.
• Removed the private helpers matchesFactorType(...) and getEffectiveType(...).
• No endpoint or public signature change — internal filtering behavior only.
• Behavioral change: callers passing channel-level/wildcard values (sms, oob) must now pass the resolved type value (e.g. phone). The values map one-to-one to MfaFactor.type from MfaRequirements.

2. TotpEnrollmentChallenge — correct field mapping for /mfa/associate

• Made id/authSession nullable and added authenticatorType, secret, recoveryCodes, so a single class deserializes both /mfa/associate (ROPG MFA) and /authentication-methods (My Account).
• Fixes a null-safety bypass (id/authSession were declared non-null but arrive null from /mfa/associate) and stops silently dropping secret/recovery_codes.
• My Account TOTP flow unaffected — its fields still map identically; the change only loosens nullability and adds fields.

Docs

• EXAMPLES.md: MFA factor examples updated (sms → phone) and TOTP enroll snippets read secret (not manualInputCode) and show recoveryCodes, for both Kotlin and Java.

Test plan

• ./gradlew :auth0:testDebugUnitTest --tests "com.auth0.android.authentication.MfaApiClientTest" — 58 tests green (fixtures updated to type-based matching + /mfa/associate TOTP shape)
• Full :auth0 suite — ./gradlew :auth0:testDebugUnitTest — all green; MyAccountAPIClientTest (42) confirms the shared TotpEnrollmentChallenge change does not regress the My Account TOTP flow

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