Fix --json-path scoping and validation for URL inputs

[vmvarela]

Problem

--json-path already selects the nested array used as rows for JSON input, but its current validation does not correctly account for per-URL --input-format.

This valid command can fail because -I json is attached to the preceding URL while --json-path validation checks only the global input format:

sql-pipe --url https://api.example.com/items -I json --json-path data.items 'SELECT * FROM url0'

--json-path is also global, while --input-format and --http-header support per-URL configuration.

Proposed Behavior

• --json-path after a --url applies to that URL.
• --json-path before any URL remains the global value for files and stdin.
• A URL-specific path overrides the global path.
• Different JSON URLs may use different row-array paths.
• The path remains simple dot-separated key navigation, not full JSONPath.

sql-pipe --url users=https://api.example.com/users -I json --json-path data.users --url orders=https://api.example.com/orders -I json --json-path results.orders 'SELECT * FROM users JOIN orders ON users.id = orders.user_id'

Acceptance Criteria

• Store an optional JSON path in each UrlInput.
• Pass the URL-specific path to the JSON loader.
• Fall back to the global path when no URL-specific path exists.
• Accept --url ... -I json --json-path ....
• Reject URL-specific paths for explicitly non-JSON URL inputs.
• Preserve existing behavior when --json-path is omitted.
• Add integration tests for one URL, multiple URLs, fallback, and invalid formats.
• Update help, completions, README, and man page.
• Do not add full JSONPath, flattening, or column-level path mappings.

References

• feat: add --json-path flag to navigate nested JSON arrays for input #141 introduced --json-path.
• feat: support multiple --url arguments with per-URL format and headers #211 introduced per-URL format and header options.