Self-describing query envelope

[spascarella]

Implemented the self-describing query envelope.

• Single queries now include data.shape: "columnar".
• Batch queries include data.shape: "columnar" and data.results[].shape: "columnar".
• Existing columns, types, and cells layout is unchanged.
• Updated contract documentation and changelog.
• Added regression coverage for single and batched responses.
  Validation:
• Query tests: 43 passed
• Safety and billed-query tests: 225 passed
• Ruff lint and formatting checks passed

Related issues

Closes #282.

[marcociav-exmergo]

LGTM