feat(types): widen SdkBeta to accept arbitrary anthropic-beta values

[abhicris]

Closes #845.

What

SdkBeta is currently Literal["context-1m-2025-08-07"], so type checkers reject any other beta flag — even though the runtime path in subprocess_cli.py:

if self._options.betas:
    cmd.extend([\"--betas\", \",\".join(self._options.betas)])

is already happy to forward arbitrary strings. That blocks callers from opting into header-only betas like token-efficient-tools-2025-02-19, which on tool-heavy agent loops materially reduces output tokens (~14% on the workloads the issue cites).

Why this approach

The issue (#845) raises two options. This PR takes option 1 (loosen the literal) because it is purely additive — no existing usage breaks, no runtime behavior changes — and the Literal arm keeps documenting the value the SDK explicitly supports today. Option 2 (extra_headers) is a larger surface change and worth keeping for a follow-up if maintainers want a separate channel for non-anthropic-beta headers.

Diff shape

• src/claude_agent_sdk/types.py: SdkBeta = Literal[\"context-1m-2025-08-07\"] | str plus a couple of comment lines explaining the intent. Updated the betas field docstring on ClaudeAgentOptions to spell out that arbitrary header values are accepted, with token-efficient-tools-2025-02-19 as the canonical example.
• tests/test_transport.py: two new _build_command tests:
  • test_build_command_forwards_known_beta — guards the existing literal value end-to-end.
  • test_build_command_forwards_arbitrary_beta — passes a known + unknown beta side-by-side and asserts they reach --betas in order, joined as the CLI expects.

+51 / -2. The widened alias compiles to the same wider str at runtime, so type checkers will see betas: list[str]. The Literal member remains useful for IDE autocomplete on the value the SDK officially documents.

Test plan

• ruff check and ruff format --check clean on the touched files.
• pytest tests/test_transport.py tests/test_types.py — 130 passed (the 2 unrelated failures both also fail on main for me, they need a trio backend not relevant to this change).
• Manual: passed betas=[\"context-1m-2025-08-07\", \"token-efficient-tools-2025-02-19\", \"some-future-beta-2099-12-31\"] through ClaudeAgentOptions constructor — runtime accepts; new test asserts the comma-joined value reaches --betas exactly.

Notes for reviewers

• The known literal stays in place, so anyone relying on the autocomplete signal still sees \"context-1m-2025-08-07\" highlighted.
• Existing callers that pass only the known literal are unaffected at type-check time; type narrowing on list[SdkBeta] continues to admit the literal.
• If this lands, follow-up [FEATURE] Allow opting into additional anthropic-beta headers (e.g. token-efficient-tools-2025-02-19) #845 (option 2 — extra_headers for non-anthropic-beta headers) could be pursued separately if there's a need.