refactor: build Iceberg test metadata at runtime

[alexanderbianchi]

Tests can supply complete Iceberg table metadata, explicit table options, and in-memory fixture files through IcebergTestHarness::builder().

• with_table_metadata accepts native Iceberg metadata without reconstructing its IDs, schemas, or snapshot history.
• with_table_option replaces duplicated registration SQL in the selected-snapshot and invalid-snapshot-ID tests.
• with_file(uri, bytes) overrides a fixture file, including manifests and manifest lists. Explicit files take precedence over generated metadata; unmodified paths fall back to the checked-in taxi fixture. An empty-table registration test verifies raw metadata override behavior.

build() now constructs the session and registers the table directly; there is no intermediate create() helper. The harness remains usable without distributed integration features.

Shared fixture helpers expose only the original taxi metadata and a native metadata builder starting before the first snapshot. Snapshot recipes stay test-local. The duplicated missing-summary JSON fixture is removed.

Rebased onto iceberg-0.10 at 6507bf8, including merged #687. The table-options cleanup and file-override support are separate commits. Test-case consolidation is intentionally kept in an independent PR.

Validation on #700 at bb3872e:

• cargo test -p datafusion-distributed-iceberg --locked — 90 tests passed, including the doctest
• cargo clippy -p datafusion-distributed-iceberg --all-targets --locked -- -D warnings
• cargo fmt --all -- --check
• git diff --check

[alexanderbianchi]

First iteration required a duplicated check in each function above, i'd rather keep them 1 line and isolate the input to one place. If you think a struct for this is overkill happy to split it up into the impl Storage

[alexanderbianchi]

If we could do like .into_builder() on the current snapshot this wouldn't be a problem but the LoC for a new snapshot are needed right now.

[alexanderbianchi]

Should we consider something like

   async fn run(
       ctx: &SessionContext,
       query: &str,
   ) -> Result<(Arc<dyn ExecutionPlan>, Vec<RecordBatch>)> {
       let df = ctx.sql(query).await?;
       let plan = df.create_physical_plan().await?;
       let batches = collect(Arc::clone(&plan), ctx.task_ctx()).await?;
       Ok((plan, batches))
   }

from tests/multi_task_collect_join_repros.rs.

[gabotechs]

👍 no blockers, mainly just questions