[AMORO-1604][Iceberg] Preserve string event-time metrics in mixed-table writers

[pallavithegod]

Why are the changes needed?

Close #1604.

A string event-time field can inherit Iceberg's default truncate(16) metrics mode. Timestamp strings such as 2026-10-16 12:34:56 need more than 16 characters, so a truncated upper bound cannot reliably drive watermark calculation.

Default the configured string event-time column to full metrics in BasicUnkeyedTable.properties(), after catalog properties are merged. Mixed-table writers consume these effective properties for both newly created and existing tables. This covers the table-loading path rather than only internal-catalog creation. Explicit per-column metrics settings remain authoritative, and unrelated columns keep their existing behavior.

Brief change log

• Supply write.metadata.metrics.column.<event-time-field>=full when the event-time column is a string and has no explicit column override.
• Leave timestamp-typed, absent, and unknown fields unchanged.
• Avoid changing the source property map or persisted metadata.
• Document the effective-writer scope: existing file bounds are not repaired, and writers accessing the underlying Iceberg table directly must configure metrics separately.
• Add real Parquet/ORC bounds checks, Avro writer configuration coverage, catalog defaults, and explicit override cases.

How was this patch tested?

• Added new unit test cases that verify the changes (TestEventTimeMetrics, 12 cases).
• Verified existing unit tests pass locally (TestWatermarkGenerator and TestTableWatermark); 41 selected tests passed in total.
• Tested manually in local environment.

Using Java 11 and Maven 3.9.11:

• ./mvnw spotless:apply checkstyle:check passed.
• ./mvnw test -pl amoro-format-iceberg -am -Dtest=TestEventTimeMetrics,TestWatermarkGenerator,TestTableWatermark -Dsurefire.failIfNoSpecifiedTests=false passed in Linux.
• The 12 new cases also passed on Windows.
• Against unmodified master, the new suite had four failures and no errors.