fix: reject cross-segment ExclusiveStartKey in parallel Scan

[LeeroyHannigan]

What

Reject a cross-segment ExclusiveStartKey in a parallel Scan.

A parallel Scan (TotalSegments/Segment) accepted an ExclusiveStartKey belonging to a different segment, silently returning a truncated or empty page. This validates the start key against the same segment-assignment function used
for the scan and rejects a mismatch with a ValidationException naming the correct Segment, so a key returned as a LastEvaluatedKey for one segment is only valid when the same segment is re-scanned.

Why

DynamoDB validates that an ExclusiveStartKey supplied to a parallel scan belongs to the requested segment, and rejects it otherwise; the engine instead accepted any key and combined it with the segment predicate, silently yielding a truncated or empty page rather than an error. This is a correctness/parity gap for parallel-scan pagination. The behavior was verified against the real DynamoDB service.

Closes #

Testing done

• Scan integration test: a key that belongs to a segment is accepted as that segment's ExclusiveStartKey, and rejected with a ValidationException when supplied for a different segment.
• Verified verbatim against the real DynamoDB service (us-east-1): a same-segment ExclusiveStartKey paginates within the segment; a cross-segment key returns ValidationException: The provided starting key is invalid: Invalid ExclusiveStartKey. Please use ExclusiveStartKey with correct Segment. TotalSegments: 4 Segment: N.
• cargo test -p extenddb-storage-postgres unit pass; scan integration test passes; cargo clippy --workspace --all-targets and cargo fmt --all -- --check clean.

Checklist

• I have read CONTRIBUTING.md
• All tests pass (cargo test --workspace)
• Code is formatted (cargo fmt --check)
• Clippy is clean (cargo clippy -- -W clippy::pedantic)
• I have added or updated tests for new functionality
• I have updated documentation if behavior changed
• Breaking changes are noted below (if any)
• If this changes the wire protocol, Storage trait, auth model, on-disk
  format, or public CLI surface, an RFC has been accepted or is linked
  below. Otherwise, an ADR captures the decision (link below).

ADR / RFC: n/a -- no change to wire protocol, Storage trait, auth model,
on-disk format, or public CLI surface. Request-validation correctness only.

Breaking changes

None. A request that was previously mis-accepted (a cross-segment ExclusiveStartKey) now returns the DynamoDB-correct ValidationException; valid same-segment pagination is unchanged.

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[jcshepherd]

It's a little bizarre that we're executing a SQL query to do a mathematical calculation. I get why but in a perfect world I wonder if we'd be taking a dependency on a Postgres internal hash function for assigning segments.