MCP query_metrics accepts embedded NUL prefixes in time_group #24371

The MCP query_metrics handler reads time_group through json_object_get_string() and compares/parses the resulting C string. An embedded NUL therefore hides the suffix: "time_group": "max\u0000junk" reaches the same grouping selection as "max" when the other query arguments are valid.

Source trace checked against netdata/netdata @ b97c83c and PR #23281 head 6e5d82d:

• src/web/mcp/mcp-tools-query-metrics.c: JSON string extraction and C-string comparisons in the grouping-validation block; grouping conversion later passes that same string to time_grouping_parse() (current lines 396-409 and 543-544).
• The behavior already exists on master. PR api: fleet time-aggregations - percentage-of-time, number-of-flaps, number-of-times #23281 validates complete condition/numeric option strings, but does not introduce identifier truncation.

Reproduction input: use any valid existing query_metrics request with time_group set to "max\u0000junk"; the encoded JSON string is longer than the method name accepted by the C-string parser. This report is based on the source trace, not a captured live-server response.

Expected: validate the complete JSON method identifier and return a structured parameter error for an embedded NUL. Add regression coverage for NUL before, inside and after a valid name, with an ordinary valid-name control. Check other enum parameters separately before expanding the scope.

Discovered while reviewing #23281 (review comment 4307885353). This is independent of that PR's expression/options contract and is not a merge dependency.