[BUG] export_code: forecaster-shaped fit example for all scitypes, is_pipeline false-positives, ignored dataset param, fails on loaded models #534

Severity: P1 — the generated artifact is wrong or unusable. Verified live 2026-08-09. Note: the string-escaping bug (BUG-02) is already fixed (commit c6b2fd7); these are the remaining export_code defects.

BUG-03 — fit example is forecaster-shaped for every scitype

export_code(handle, include_fit_example=true) always emits {var}.fit(y) + {var}.predict(fh=fh), regardless of object_type:

export_code(<BoxCoxTransformer>, include_fit_example=true)  -> runs, then AttributeError: no attribute 'predict'
export_code(<SlidingWindowSplitter>, include_fit_example=true) -> runs, then AttributeError: no attribute 'fit'

Fix: branch the example on the object_type tag — transformers get fit_transform(X), splitters get list(splitter.split(y)), classifiers/regressors get fit(X, y) + predict(X).

BUG-04 — is_pipeline heuristic false-positives on any list argument

is_pipeline = "*" in spec or "Pipeline" in spec or "[" in spec

SlidingWindowSplitter(window_length=24, fh=[1,2,3]) reports is_pipeline: true because of the [.
Fix: inspect the instantiated object (isinstance / object_type tag), not the spec string.

NB-15 — dataset parameter is ignored; fit example always hardcodes load_airline

export_code(<fitted NaiveForecaster>, dataset="sunspots", include_fit_example=true)
  -> emitted code contains `from sktime.datasets import load_airline`

Data-handle ids are also silently accepted as dataset and still produce airline. An estimator fitted via y_handle gets a fabricated airline example with no provenance note.
Fix: honor the documented param; reject unknown dataset names / handle ids, or state the example dataset in the output.

NB-17 — loaded models can't be exported

load_model(path=<saved model>)  -> handle with params: {}
export_code(<that handle>)      -> {"success": false, "error": "No craft spec found in handle parameters."}

load_model handles carry empty params, so codegen has no spec to work from.
Fix: reconstruct a spec from get_params() (or repr) at load time so loaded models are exportable.

(Catalogued as BUG-03 / BUG-04 / NB-15 / NB-17 in MCP_TEST_FINDINGS.md.)