perf: Disable morsel splitting for fast-count on streaming engine

[nameexhaustion]

• Fixes Disable morsel splitting in I/O sources if scans are connected directly to in-mem sink #22702

Streaming queries that collect to an in-memory DataFrame and consist only of column projections (and pl.len()) will now disable morsel splitting at supported sources.

E.g -

• Scan->InMemorySink scan_parquet().collect()
• Fast-count scan_parquet().select(pl.len()).collect()
• Simple projections scan_parquet().select("<column>", "column", ..).collect()

Equivalently when starting from InMemorySource -

• InMemorySource->InMemorySink LazyFrame().collect()
• Fast-count LazyFrame().select(pl.len()).collect()
• Simple projections LazyFrame().select("<column>", "column", ..).collect()

Benchmark

Description: scan_parquet().select(pl.len()).collect()
File: 4B rows x 0 columns

Runtime Before	Runtime After	Speedup
0.0728s	0.000207s	351x

Before this PR the parquet source would split to morsels of 100k rows (sending ~42,949 morsels). It now sends only a single morsel.

Test script

from time import perf_counter
import polars as pl

path = "/Users/nxs/git/polars/.env/_data_out/big.parquet"
pl.LazyFrame(height=(1 << 32) - 1).sink_parquet(path)

q = pl.scan_parquet(path).select(pl.len())
print(q.explain(engine="streaming"))

timings = []
for _ in range(5):
    t = perf_counter()
    q.collect()
    timings.append(perf_counter() - t)

print(f"{min(timings) = }")

[codecov]

Codecov Report

❌ Patch coverage is 99.15254% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 81.12%. Comparing base (5dd9b23) to head (889ab84).

Files with missing lines	Patch %	Lines
...ates/polars-stream/src/nodes/io_sources/ipc/mod.rs	91.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #26245      +/-   ##
==========================================
+ Coverage   78.35%   81.12%   +2.76%     
==========================================
  Files        1777     1777              
  Lines      241720   241816      +96     
  Branches     3085     3085              
==========================================
+ Hits       189406   196172    +6766     
+ Misses      51517    44848    -6669     
+ Partials      797      796       -1     

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