perf(mongodb): single-pass tokenizer in MqlParser._normalizeJsObject eliminating repeated string copying #1539

Problem

In MqlParser._normalizeJsObject (lib/features/mongodb/mql_parser.dart):

1. Runs three replaceAllMapped passes with dynamic RegExps.
2. Replaces string literals with placeholders (@@@STR_0@@@, @@@STR_1@@@).
3. Iterates over placeholders with processed = processed.replaceAll('@@@STR_$i@@@', strings[i]).
   For a query containing 50 strings, replaceAll creates 50 complete copies of the entire query string in heap.

Solution

• Replace the multi-pass placeholder mechanism with a single-pass tokenizer/scanner streaming normalized JSON into a StringBuffer.
• Quote keys and preserve string literals in one linear scan (N)$ without intermediate query re-allocations.

Benefits

• Single linear pass (N)$ with zero repeated string cloning.
• Significantly faster MQL parsing for batch inserts and complex queries.