fix(cloudformation): keep CloudFront CustomErrorResponses through provisioning

[Sorttech]

Two fidelity mismatches on the same block, both of which left a
CFN-provisioned SPA distribution with no deep-link fallback.

Translating the CFN block ran each rule through serde_json::from_value
into the wire struct and dropped anything that failed. CloudFormation
types ResponseCode as an Integer while the CloudFront API carries it as
a string, so every rule failed to deserialize and the distribution came
out with Quantity: 0. Map the fields explicitly instead, coercing the
number and accepting the quoted form YAML templates and resolved
intrinsics produce. A rule missing the required ErrorCode is skipped
rather than defaulted to a code that would match nothing.

ErrorCachingMinTTL then still failed to round-trip: rename_all = "PascalCase" renders it ErrorCachingMinTtl, so the field was mis-named
on the wire and dropped on parse from real SDK requests. Pin the AWS
spelling, as WebACLId in the same file already does.

Test plan

Regression test: cfn_provisions_spa_custom_error_responses in crates/fakecloud-e2e/tests/cloudformation_cloudfront.rs

Verification

On this exact head, rebased onto current main:

• cargo fmt --all --check - clean.
• cargo clippy --workspace --all-targets -- -D warnings - clean.
• cargo test --workspace excluding fakecloud-e2e, fakecloud-conformance,
  fakecloud-tfacc and fakecloud-parity (the same set CI's test job runs),
  plus cargo test -p fakecloud-conformance --lib - 9836 passed, 1 failed
  (--no-fail-fast, --test-threads=4). The failure is in fakecloud-ssm
  (service::tests::send_command_completion_persists_success); an earlier run
  of the same command failed its sibling
  rearm_in_flight_commands_settles_restored_pending instead. Both read the
  last durable snapshot right after the in-memory status flips to Success
  while the save is asynchronous, and they fail 4 of 11 runs on untouched
  main on this machine. This branch does not touch SSM.
• cargo test -p fakecloud-e2e --test cloudformation_cloudfront - the
  regression test above passes against a freshly built target/debug/fakecloud.

Found by deploying a CDK CloudFront + S3 SPA against fakecloud.

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Summary by cubic

Fixes CloudFormation provisioning so CloudFront distributions keep their CustomErrorResponses (the SPA deep-link fallback) instead of provisioning with Quantity: 0.

Two mismatches caused the loss:

• CloudFormation types ResponseCode as an Integer while the CloudFront API carries it as a string, so translating each rule through serde_json::from_value into the wire struct failed and silently dropped every rule. Fields are now mapped explicitly, coercing the number and accepting quoted values from YAML templates and resolved intrinsics.
• ErrorCachingMinTTL was emitted as ErrorCachingMinTtl by rename_all = "PascalCase" and dropped on parse; the AWS spelling is now pinned, as WebACLId already is.

Rules missing the required ErrorCode are skipped rather than defaulted to a code that would match nothing. Adds a regression test.

^{Written for commit a951ab7. Summary will update on new commits.}