refactor(ssm): split long parameter, document, command, and maintenance ops

[vieiralucas]

Summary

Four SSM operations were carrying enough inline parsing/duplication to obscure their actual work.

• parameters: `get_parameter` and `get_parameters` both built the same 'historical version Value' inline (same `json!` body, same SecureString masking, repeated three times across the two functions). Move that into `build_param_history_value` so each call site is one line.
• documents: `get_document` had a 35-line if/else chain to resolve which `SsmDocumentVersion` the request was asking for (`VersionName` + `DocumentVersion` combinations, plus `$LATEST`, plus default). Move it into `select_document_version`, a pure function over `(doc, version, version_name)` that returns `Option<&SsmDocumentVersion>`.
• commands: `send_command`'s first 80 lines were entirely body parsing and validation. Move them into `SendCommandInput::from_body` so the operation body just constructs `SsmCommand`, persists it, and shapes the response.
• maintenance: same treatment for `create_maintenance_window` — pull the field-by-field validation and parsing into `CreateMaintenanceWindowInput::from_body`. The operation body keeps the idempotency check + state insertion which is the part that actually mutates state.

No behavior change. Same lock semantics, same error messages.

Test plan

• `cargo fmt`
• `cargo clippy --workspace --all-targets -- -D warnings`
• `cargo test -p fakecloud-ssm` (60 passed)

---

Summary by cubic

Refactored SSM handlers to move heavy parsing and version selection into small helpers. The handlers are shorter, easier to read, and behavior is unchanged.

• Refactors
  • Parameters: added build_param_history_value to build historical values and apply SecureString masking; simplified get_parameter/get_parameters.
  • Documents: added select_document_version to handle DocumentVersion/VersionName (incl. $LATEST and default); get_document stays defaulting to JSON when format isn’t specified.
  • Commands: added SendCommandInput::from_body for request parsing/validation; send_command now focuses on constructing SsmCommand, persisting, and shaping the response.
  • Maintenance: added CreateMaintenanceWindowInput::from_body; operation keeps the ClientToken idempotency check and state insertion.

^{Written for commit 89f9fdf. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 4 files