feat: add deploy validation CLI and improve LLM initialization ergonomics

Add crewai deploy validate to check project structure, dependencies, imports, and env usage before deploy
Run validation automatically in deploy create and deploy push with skip flag support
Return structured findings with stable codes and hints
Add test coverage for validation scenarios

refactor: defer LLM client construction to first use

Move SDK client creation out of model initialization into lazy getters
Add _get_sync_client and _get_async_client across providers
Route all provider calls through lazy getters
Surface credential errors at first real invocation

refactor: standardize provider client access

Align async paths to use _get_async_client
Avoid client construction in lightweight config accessors
Simplify provider lifecycle and improve consistency

test: update suite for new behavior

Update tests for lazy initialization contract
Update CLI tests for validation flow and skip flag
Expand coverage for provider initialization paths