Fix false positive error detection when JSON data contains ClickHouse exception text

[jradtilbrook]

Summary

• When querying tables that store ClickHouse exceptions as data (e.g. error tracking systems), hasErrorClickhouse() can falsely flag valid responses as errors if the exception text appears in the last 4096 bytes of the response body.
• On PHP 8.3+, uses json_validate() as a confirmation step when the tail-regex matches: if the response is structurally valid JSON, the query succeeded and the match is just data.
• Zero change in behaviour on PHP < 8.3. Zero overhead on the common path (regex doesn't match).

Problem

The > 4096 byte branch in hasErrorClickhouse() checks the last 4096 bytes of a JSON response against CLICKHOUSE_ERROR_REGEX. This exists to detect mid-stream errors (where ClickHouse appends error text to an already-streaming HTTP 200 response, breaking the JSON structure).

However, if the response data legitimately contains ClickHouse exception strings (e.g. an error tracking system storing exceptions in a ClickHouse table), and that text lands in the tail of the response, it triggers a false positive.

Solution

When the regex matches in the tail, call json_validate($body) (PHP 8.3+) before concluding there's an error:

• Valid JSON → query succeeded, the match is just data → return false
• Invalid JSON → the response structure is broken by a real mid-stream error → return true

json_validate() is O(1) memory (no decoded structure allocated) and only runs in the case where the regex already matched.

[jradtilbrook]

Related to #234