fix(storage): handle MatrixOne aggregate and nullable parameter compatibility

[loveRhythm1990]

Summary

• replace count-like SUM expressions with conditional COUNT so MatrixOne does not return DECIMAL128 for integer counts
• explicitly cast aggregate results that Memoria decodes as Rust numeric types
• preserve NULL confidence averages instead of coercing unknown values to zero
• log metrics aggregate decoding failures instead of silently reporting zero
• avoid MatrixOne 4.2 prepared-statement NULL-state leakage by using distinct INSERT SQL shapes for nullable values
• add aggregate and same-connection Some -> None -> Some regression coverage

Background

MatrixOne widened SUM(INT64/UINT64) to DECIMAL128(38,0), while affected Memoria queries still decoded those values as i64.

MatrixOne 4.2 also retains a prepared parameter NULL state across executions, so a non-NULL value following NULL can be persisted as NULL. The upstream fix is merged to MatrixOne main but is not present in the current 4.2 latest image.

Aggregate change: matrixorigin/matrixone@14c4e4d

Prepared parameter fix: matrixorigin/matrixone#26915

Validation

Validated against the current matrixorigin/matrixone:latest image with a 4 GB container:

• cargo check -p memoria-storage --tests
• storage CRUD tests: 24 passed
• session-scope API tests: 5 passed
• full workspace: 834 passed, 5 ignored across 53 suites
• git diff --check

Fixes #238

[aptend]

Blocking: the new NULL-parameter workaround changes the existing empty-string contract for session_id and superseded_by.

nullable_str() deliberately normalizes both None and Some("") to SQL NULL, and the previous single/batch insert paths used it. The new SQL-shape selection uses .is_some() and binds the original string, so Some("") now produces a bound empty string rather than a literal NULL in both paths.

Please derive the shape and bind value from nullable_str(...) (not .is_some()), then add regression coverage for empty session_id / superseded_by in both single and batch inserts.

[loveRhythm1990]

Fixed in 84415b4. Both single and batch INSERT paths now derive the SQL shape and bind values from nullable_str() for session_id and superseded_by, preserving Some("") -> SQL NULL. Added raw-database regression coverage for both fields in single and batch inserts. Validation: Rust 1.85 cargo check/clippy passed; storage CRUD 26/26; session-scope API tests 5/5.

[aptend]

Re-reviewed 84415b4. The NULL SQL-shape selection and bound values now consistently derive from nullable_str() for both single and batch INSERTs, and the new raw-database regression tests cover the empty-string contract. No remaining blocking issues.