GeezerVison

Drop the python-dateutil dependency from both the generator and all
generated client code. Date/datetime parsing now uses the stdlib:

- datetime fields: datetime.datetime.fromisoformat(v.replace("Z", "+00:00"))
- date fields: datetime.date.fromisoformat(v)

The .replace("Z", "+00:00") call is needed because Python 3.10's
fromisoformat() does not accept the Z timezone suffix (added in 3.11).
It is a no-op on strings that do not contain Z.

Default values in OpenAPI specs are normalized at generation time
(Z replaced with +00:00), so the generated default expressions are
clean datetime.datetime.fromisoformat("...") calls without the
replace.

This removes one runtime dependency from every generated client
package, reducing install size and eliminating a dependency that is
in maintenance-only mode upstream.

Make the .replace("Z", "+00:00") workaround discoverable by both
`rg TODO` and `rg "3.10|py3.10"` so it can be cleanly removed
when the project drops Python 3.10 support.

Brings in 8 commits since the PR opened, notably:
  - typer constraint bump to <0.27
  - uv_build 0.11 template update
  - mypy v2 cast cleanup in generated output

Conflict resolution:
  - pdm.lock, integration-tests/pdm.lock: regenerated via 'pdm lock'
    against the merged pyproject.toml.
  - Golden records: regenerated via 'pdm run regen' to absorb the
    mypy-cast cleanup; the dateutil -> fromisoformat change from this
    branch is preserved.

Verified locally with ruff check, ruff format --check, mypy, and the
unit-test suite (283 passed, 4 skipped).

The minimal lockfiles were previously regenerated without
'-S direct_minimal_versions', so direct dependencies were locked
to highest-compatible versions instead of lowest. That defeats the
point of the test_min_deps CI job, which exists to verify the
declared lower bounds in pyproject.toml still work.

Re-locked both pdm.minimal.lock files using:
    pdm lock -S direct_minimal_versions -L pdm.minimal.lock

Strategy marker confirmed as
["direct_minimal_versions", "inherit_metadata"] in the metadata
of both files. python-dateutil is absent (this branch's main change).

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