fix: key generated files on the notebook, not a default pipeline name

[Ya-shh]

Fixes #942.

Two composition notebooks compiled in one directory with kale --nb overwrote each other's generated files.

Cause

Wider than composition. kale --nb documents that an explicit CLI argument beats the notebook's metadata:

If the same argument (e.g. pipeline_name) is provided both in the Notebook metadata and from CLI, the CLI parameter will take precedence.

But --pipeline_name carried default="kale-pipeline". The overrides dict is built from every argument that is not None, so the default was indistinguishable from a value the user typed, and the notebook's own pipeline_name could never win. Every notebook compiled on the CLI was called kale-pipeline, which is why the issue sees notebook-sequence and notebook-sequence-mixed ignored.

For a single notebook that meant two files were overwritten. A composition generates one module per referenced notebook, so it meant five.

Fix

• --pipeline_name no longer has a default, restoring the documented precedence.
• A notebook that declares no pipeline name is named after its file, so removing the default cannot turn into a validation failure. One notebook in the repo is in that position (examples/taxi-cab-classification/read_data.ipynb).
• _module_name keys on the root notebook's file rather than its pipeline name. File names are necessarily unique within a directory; pipeline names are not, so with only the CLI fix two notebooks sharing a name would still collide.

Result

$ kale --nb main.ipynb
$ ls .kale/
kale_notebook_main_notebook_a.py  notebook-sequence.kale.py
kale_notebook_main_notebook_b.py  notebook-sequence.pipeline.yaml
kale_notebook_main_notebook_c.py

$ kale --nb main_mixed.ipynb
$ ls .kale/
kale_notebook_main_mixed_notebook_a.py  notebook-sequence-mixed.kale.py
kale_notebook_main_mixed_notebook_b.py  notebook-sequence-mixed.pipeline.yaml
kale_notebook_main_mixed_notebook_c.py  (plus main's, untouched)

Both notebooks now compile under the names their metadata declares, which the issue called out as being ignored.

Note on behaviour change

kale --nb some_notebook.ipynb no longer writes kale-pipeline.kale.py; it writes a file named after the notebook's pipeline. That is the documented behaviour rather than new behaviour, but it is a visible change for anyone who relied on the fixed name. --pipeline_name still overrides, unchanged.

Testing

• Two new tests: two compositions sharing a pipeline name keep separate modules, and a notebook without a pipeline name is named after its file.
• 335 backend tests pass.
• The reproducer set from the feat: add notebook: cell type for multi-notebook composition. #867 review is unchanged.

[google-oss-prow]

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