The `CONTEXT_1M_BETA_HEADER` (`context-1m-2025-08-07`) is deprecated for
Sonnet 4 and 4.5. This removes the constant from the anthropic crate and
the match arm in `beta_headers()` that sent it for
`ClaudeSonnet4_5_1mContext`.

Note: The bedrock crate still has its own copy of this constant, used
when the user-configurable `allow_extended_context` setting is enabled.
That may warrant a separate cleanup.

Closes AI-114

Release Notes:

- N/A