Avoid eager local timezone lookup when ClickHouse sends a timezone header

[sadokfire]

Summary

Avoid evaluating Poco::Timezone::name() when the ClickHouse response already
contains X-ClickHouse-Timezone.

Root cause

The existing call used Poco::Timezone::name() as the default argument to
response->get(). C++ evaluates that argument before calling get(), even
when the response header is present. On glibc, Poco::Timezone::name() calls
tzset() and reads tzname. Concurrent localtime_r() activity can leave
tzname[0] null between those operations, causing a crash even though the
driver then uses the server-provided timezone.

Changes

• Resolve the response timezone through a lazy fallback function, so
  Poco::Timezone::name() is called only if the header is absent.
• Add deterministic tests for present nonempty, present empty, and absent
  timezone headers. The present-header tests supply a fallback that fails if it
  is evaluated.

The change preserves the existing absent-header fallback and an explicitly
present empty header.

Validation

• Oracle Linux 9.8 (glibc 2.34): all 548 ANSI and 548 Unicode unit tests pass.
• The three Statement.ResponseTimezone* tests pass in both ANSI and Unicode
  executables.
• Fresh real UnixODBC harness run against ClickHouse 26.6.1.1193:
  • 5-second control: 4,772 queries, 0 conversions, result=0;
  • 10-second concurrent localtime_r() run: 11,794 queries,
    263,262,925 conversions, result=0.
• The unmodified driver was previously reproduced crashing with exit 139 under
  the same concurrent workload; the Docker bundle contains the complete flow.

Manual reproduction

Attach clickhouse-odbc-timezone-race-reproducer-v2.zip from this change. It
contains the Docker build environment, UnixODBC harness, expected baseline
failure, fixed-driver verification, and cleanup commands.

[CLAassistant]

All committers have signed the CLA.

[sadokfire]

clickhouse-odbc-timezone-race-reproducer-v2.zip