fix(workflow): preserve explicit null values in typed node inputs

[hulkbig]

Describe the bug

Workflow input normalization drops valid nullable fields by serializing Pydantic models with exclude_none=True. A required nullable value becomes missing before the function validates its input again, and explicit null on a field with a non-null default can silently become that default, including in nested models.

Preserve None during input normalization with a keyword-only exclude_none option on validate_node_data. Its existing True default retains output validation and other callers' behavior. Regressions cover required/defaulted/nested nulls, omitted fields, message and predecessor inputs, and model/list/dictionary serialization.

Reproduction

Define a model with value: str | None and a workflow function accepting node_input of that model type. Run Workflow(edges=[(START, consume)]) through InMemoryRunner with a user message containing {"value": null}, or pass {'value': None} from a preceding node. Before the fix, this raises Field required; a non-null field default instead replaces explicit null. After the fix, explicit null reaches the function as None, an omitted field uses its default, and an omitted required nullable field remains invalid.

The seven public-workflow regressions give four failures and three control passes with the pristine base methods; all seven pass with this change. The original built-wheel replay through InMemoryRunner also passes all seven scenarios. No model API is involved.

Current revision and verification

Head f9ec32a162deb578191142a4133f3c6d347ace39 normally merges upstream main bcac35a6efb21f176eef9d28d663db247f4e5afd. The net PR diff is the original four-file nullable-input fix (141 insertions, 3 deletions). The earlier GKE fixture correction is now present in main and no longer appears in this diff.

• Python 3.11.17, existing isolated test environment, checkout src selected explicitly: full workflow and utils suites plus GKE executor tests: 1,675 passed, 3 xfailed, zero failures/errors.
• Sdist and wheel build; changed-file Ruff, isort, Pyink, license, compliance, guide-link and spelling hooks pass. The generic staged-new-file hook also inspected upstream-only additions and reported four missing unit guides. Running the same repository checker against the actual PR's added-file set passes (there are no new production files). No waiver or hook change was added.
• git diff --check passes.

Earlier full Python 3.11–3.14 matrix evidence was collected at dad5ca17894ad4a0c34962326db118d24e7f0181; later 6,814-test scoped checks per version were collected at b80d408ba012cdcc7a786ca6e249d832c7a0a3bc. These are historical results, not full-matrix validation of this new head. The current sync is covered by the focused execution above; hosted CI and maintainer review remain pending.

AI assistance was used for implementation, review and validation.