fix: [iceberg] Remove IcebergFileStream and use iceberg-rust's parallelization

[mbutrovich]

Which issue does this PR close?

N/A

Rationale for this change

Profiling Iceberg native scans revealed significant overhead in async stream polling, particularly:

• tokio::drop_waker and tokio::park::clone consuming substantial time in IcebergStreamWrapper::poll_next
• futures_util::stream::flatten_unordered::SharedPollState::{start_polling,stop_polling} showing lock contention

I think this is due to:

1. Per-batch schema adapter allocation: Created SparkParquetOptions, SparkSchemaAdapterFactory, and schema adapters for every single batch via .and_then() combinator
2. Competing parallelization logic: IcebergFileStream passed one FileScanTask at a time to iceberg-rust, causing flatten_unordered to coordinate parallelization across a single task (pure overhead). Stream nesting created excessive waker churn.

What changes are included in this PR?

• Cache schema adapters and Parquet options
• Remove IcebergFileStream and pass all FileScanTasks directly to iceberg-rust. I tried this in the past but I can't remember why I abandoned it. Let's try again.

How are these changes tested?

Existing tests.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.47%. Comparing base (f09f8af) to head (f17099a).
⚠️ Report is 834 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #3051      +/-   ##
============================================
+ Coverage     56.12%   59.47%   +3.34%     
- Complexity      976     1382     +406     
============================================
  Files           119      167      +48     
  Lines         11743    15512    +3769     
  Branches       2251     2575     +324     
============================================
+ Hits           6591     9226    +2635     
- Misses         4012     4989     +977     
- Partials       1140     1297     +157     

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