refactor: extract shared Query protocol helpers to fakecloud-core

[vieiralucas]

Summary

• Adds fakecloud_core::query module with shared Query protocol XML wrapping (query_response_xml, query_metadata_only_xml) and param extraction (optional_query_param, required_query_param)
• Migrates SQS, ElastiCache, RDS, SES v1 to delegate to the shared implementations, eliminating 4 identical xml_wrap() copies and 2 identical optional_param/required_param pairs
• Makes SQS md5_hex/sha256_hex pub(crate) since they're only used within the crate

Test plan

• cargo clippy --workspace --all-targets -- -D warnings passes clean
• Existing xml_wrap_produces_valid_response test in ElastiCache still passes
• CI passes
• Cubic review passes

---

Summary by cubic

Extracted shared AWS Query protocol helpers into fakecloud-core and updated fakecloud-sqs, fakecloud-elasticache, fakecloud-rds, and fakecloud-ses (v1) to use them. Removes duplicate XML wrappers and parameter parsing; no behavior change intended.

• Refactors

  • Added fakecloud_core::query::{query_response_xml, query_metadata_only_xml, optional_query_param, required_query_param}.
  • Replaced per-service xml_wrap() and param helpers in the services to delegate to core.
  • Set fakecloud-sqs md5_hex/sha256_hex to pub(crate) as they are internal utilities.

• Migration

  • If external code used fakecloud-sqs::md5_hex or sha256_hex, switch to md5/sha2 directly or copy the small helpers.

^{Written for commit 7610253. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 6 files