fix: install MEOS error handler so errors throw instead of exit()

[estebanzimanyi]

Summary

MEOS's default error handler writes the message to stderr and calls exit(EXIT_FAILURE) on errlevel == ERROR (PG elog level 21, carried through via postgres.h). That tears down the entire DuckDB process on any invalid user input — a malformed tstzset literal, a negative precision on asText, etc. — which is both user-hostile in production and made every statement error test in PR #6's parity harness kill the test runner before Catch2 could check it.

This PR installs MobilityduckMeosErrorHandler via meos_initialize_error_handler(...) in LoadInternal, right after meos_initialize(). The handler throws DuckDB::InvalidInputException on errlevel >= 21 (ERROR/FATAL/PANIC), so MEOS errors surface as ordinary query failures with the MEOS message attached. Lower levels (WARNING/NOTICE/INFO) are silently ignored for now — can be wired to DuckDB's logger in a follow-up if useful.

Parity file consequences

• Deletes the top-of-file error-handler gap note (now obsolete).
• Unwraps the 3 MEOS-error-specific mode skip blocks covering tstzset parse errors and asText negative-digits — they now run as real statement error assertions.
• Re-wraps one of those (set('{}'::timestamptz[])) in a fresh skip block with a new gap note: that query fails at the DuckDB cast layer with a Conversion Error before MEOS sees it, because '{}' is PG array-literal syntax that DuckDB rejects as a string literal. The equivalent DuckDB form set(ARRAY[]::TIMESTAMPTZ[]) reaches MEOS but the constructor wrapper in src/temporal/set_functions.cpp returns null instead of calling meos_error(...) on empty input — a separate gap, documented for a later follow-up.

Test plan

• TZ=UTC ./build/release/test/unittest \"<proj>/test/*\" locally: 747 assertions pass across 13 test cases (up from 739 pre-handler), no regressions.
• CI validates on linux_amd64, linux_arm64, macos, wasm.
• After merge: the empty-array constructor follow-up (documented in the new skip block) unblocks that one remaining statement error.

Notes

• Throwing a C++ exception through a C callback is technically UB by the standard, but is well-defined in GCC/Clang with -fexceptions enabled (which DuckDB and its extensions use). This is the same mechanism DuckDB-spatial uses for GDAL error handlers — precedent in the surrounding ecosystem.
• errcode is currently unused in the handler. Could be mapped to more specific DuckDB exception types in a follow-up (e.g. MEOS_ERR_WKB_INPUT → InvalidTypeException), but one-size-fits-all InvalidInputException is fine as a baseline.

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