fix: use jemalloc as a default allocator inside DuckDB.

[drrtuy]

Use jemalloc as the default allocator for DuckDB

What

The DuckDB engine was compiling the glibc malloc() path in allocator.cpp even though libjemalloc_extension.a was being linked, because DUCKDB_EXTENSION_JEMALLOC_LINKED was only defined in the extension/ scope and never reached src/. This wires jemalloc in as the actual default allocator.

Key changes

• duckdb.cmake: define DUCKDB_EXTENSION_JEMALLOC_LINKED=1 globally and add the jemalloc header include dir so the USE_JEMALLOC branch compiles in duckdb_static.
• duckdb_extensions.cmake: load the jemalloc extension alongside core_functions, icu, json.
• errmsg-utf8.txt: add DuckDB engine error codes (ER_DUCKDB_*) appended at configure time for comp_err.
• disabled.def: re-enable the duckdb_time_func test.

How to test

Build the DuckDB plugin and confirm jemalloc symbols (duckdb_je_*) are present; run the DuckDB mysql-test suite, including the re-enabled duckdb_time_func.

[gemini-code-assist]

Code Review

This pull request enables the jemalloc extension for DuckDB by loading it in the CMake configuration and defining the necessary preprocessor flags. The reviewer correctly pointed out that overriding CMAKE_CXX_FLAGS directly in duckdb.cmake will discard existing compiler flags, such as optimization and debug flags, and suggested appending the new flags to ${CMAKE_CXX_FLAGS} instead.

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