Reduced scope of CLAUDE_CODE_DISABLE_EXPERIMENTAL_BETAS for certain API betas

The internal function that checks whether a user is on a first-party provider (Anthropic direct, Anthropic AWS, or Foundry) was split into two: a pure provider check and a combined provider-plus-env-var check. Two code paths were updated to use the provider-only check, meaning:

• One specific SDK beta flag (used in API request construction) is now always included for first-party provider sessions, even when CLAUDE_CODE_DISABLE_EXPERIMENTAL_BETAS=1 is set.
• The SDK beta filter function, which decides whether to pass all beta strings through or restrict to a known set, now passes all betas for first-party providers regardless of the env var.

Other uses of CLAUDE_CODE_DISABLE_EXPERIMENTAL_BETAS (e.g., gating thinking summaries, context management beta, tool schema betas) remain unchanged and continue to respect the environment variable.