feat(native-rust): encrypt behavior

[lucasmcdonald3]

Implements encrypt behavior per client-apis/encrypt.md.

Scope:

• esdk/src/encrypt.rs — encrypt entry point, default-CMM wrapping, encryption-context validation, body and signature construction, output assembly.
• Tests:
  • esdk/tests/test_encrypt_behavior.rs — top-level encrypt behavior.
  • esdk/tests/test_encrypt_missing_annotations.rs — coverage for spec requirements not directly tied to a sub-routine.
  • esdk/tests/test_construct_the_signature.rs — signature construction.
  • esdk/tests/test_cmm_algorithm_suite_override.rs — algorithm-suite override behavior.
  • esdk/tests/test_post_cmm_validation.rs — post-CMM-call material validation.
  • esdk/tests/test_keyring_to_default_cmm.rs — keyring-to-default-CMM wrapping.
  • esdk/tests/test_required_encryption_context.rs — required encryption context handling.

The public input/output types (EncryptInput, EncryptOutput) and streaming traits used by this entry point are reviewed in #919. Header, body, body-AAD, and footer serialization called from here are reviewed in their own PRs (#909, #918, #900, #898). Decrypt behavior is reviewed separately in a follow-up PR.

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Note: CI does not run on this PR. This branch is scoped for focused review and intentionally omits test helpers, scaffolding, and other supporting code. Full code — including helpers, test infrastructure, and working tests — lives on the aws-crypto-rust/unreviewed branch.

If you want more context or to actually run the tests, see aws-crypto-rust/unreviewed.

Related spec:

• client-apis/encrypt.md