refactor(kms): split create_key / encrypt / verify / create_alias

[vieiralucas]

Summary

Apply the same `*Input::from_body` + per-phase helper pattern that PRs #329 (RDS) and #347 (RDS) used, to the four long KMS functions.

• `create_key`: extract `CreateKeyInput::from_body` for the 30-line parse + validate block (length checks, enum checks, key-spec validation, tag parsing). The remainder reads as state-mutation phases.
• `encrypt`: extract `decode_plaintext` (length validation 1..=4096) and `build_encrypt_ciphertext` (the imported-material XOR vs. fakecloud envelope branch).
• `verify`: extract `require_non_empty_b64` (shared by Message + Signature), `validate_key_usage_signing`, and `validate_signing_algorithm`.
• `create_alias`: extract `validate_alias_name` (prefix / reserved `aws/` / invalid characters / regex pattern) and `validate_alias_target` (alias-of-alias rejection); add a shared `require_string_field` for the AliasName/TargetKeyId required-string boilerplate.

Test plan

• `cargo fmt`
• `cargo clippy --workspace --all-targets -- -D warnings`
• `cargo test -p fakecloud-kms --lib` — 50 passed
• `cargo test --workspace --exclude fakecloud-e2e --exclude fakecloud-conformance`

---

Summary by cubic

Refactored KMS handlers to split long functions into small, focused helpers for clearer flow and reuse, with no behavior changes. Affects create_key, encrypt, verify, and create_alias in crates/fakecloud-kms/src/service.rs.

• Refactors
  • create_key: added CreateKeyInput::from_body for parse/validate (lengths, enums, key-spec, tags); remaining code focuses on state changes.
  • encrypt: added decode_plaintext (1..=4096 bytes) and build_encrypt_ciphertext (imported-material XOR vs. fake envelope).
  • verify: added require_non_empty_b64, validate_key_usage_signing, validate_signing_algorithm.
  • create_alias: added require_string_field, validate_alias_name (prefix/reserved/regex), validate_alias_target (no alias-of-alias).
  • Centralizes validation logic; keeps error messages and behavior unchanged.

^{Written for commit 18cb646. Summary will update on new commits.}

[cubic-dev-ai]

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