chore: Simplify enum types (TtsLanguage, SpeechModel)

[Kavii-Camb]

Summary

• Regenerated SDK code from Fern with simplified enum type names
• CreateStreamTtsRequestPayloadLanguage is now TtsLanguage (moved to types/ package)
• CreateStreamTtsRequestPayloadSpeechModel is now SpeechModel (moved to types/ package)
• Includes new async client classes and other Fern-generated updates (query string mappers, nullable support, etc.)

Test plan

• Verify the SDK compiles successfully
• Confirm TtsLanguage and SpeechModel are usable from consumer code
• Check that no references to old enum names remain

[Kavii-Camb]

Closing - full regeneration caused too many unrelated changes. Will redo with surgical enum-only changes.