fix(python): default show_graph physical plan to the streaming engine

[binaydhakal]

Closes #28802

Problem

LazyFrame.show_graph(plan_stage="physical") uses the default auto engine, which plans (and, by default, executes) via the streaming engine. But _AutoEngine.plan_engine is the literal "auto", so show_graph's physical-plan branch never matched == "streaming" and fell through to the in-memory renderer. The result: the default plot differed from engine="streaming", even though auto runs the streaming plan.

import polars as pl
lf = pl.LazyFrame({"a": [1, 2, 3]})
lf.show_graph(plan_stage="physical")                     # in-memory plan (wrong)
lf.show_graph(plan_stage="physical", engine="streaming") # streaming plan

Fix

Inside show_graph, resolve auto → streaming when choosing how to build and render the plan, so the default matches the engine that actually runs. The change is scoped to show_graph: _AutoEngine.plan_engine is intentionally not touched, because collect/explain read it to select optimization flags, and broadening it there disables in-memory-only optimizations (e.g. join collapsing) for auto — verified locally that doing so regresses tests/unit/lazyframe/test_optimizations.py::test_collapse_joins.

As a drive-by (per @TNieuwdorp's note on the issue), an invalid plan_stage now raises ValueError instead of TypeError — it's an invalid value of a valid type.

Tests

• test_show_graph_phys_auto_defaults_to_streaming — asserts the auto physical plot equals streaming's and differs from in-memory's. Fails on main, passes with this change.
• test_show_graph_invalid_stage updated to expect ValueError.

Verified locally (polars built from this branch): collect/explain outputs are unchanged across all three engines, and the full tests/unit/lazyframe/ suite passes apart from 4 pre-existing serde/pickle failures unrelated to this change.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.55%. Comparing base (9b5d73f) to head (08d5897).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #28826      +/-   ##
==========================================
- Coverage   81.55%   81.55%   -0.01%     
==========================================
  Files        1888     1888              
  Lines      267844   267845       +1     
  Branches     3257     3257              
==========================================
- Hits       218445   218441       -4     
- Misses      48555    48560       +5     
  Partials      844      844              

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