Fix abstract schemas generated by provider plugin script

[fallintoplace]

Description

Fixes #505
Related to #99

Type: Bug fix

Summary

The provider plugin generator added in #144 still emits the old
supports_strict_mode property. When BaseSchema moved to the
requires_raw_output contract in #239, the generator template was not updated.
As a result, schemas created with --with-schema remain abstract and
from_examples() raises a TypeError.

This change:

• replaces the stale property with the required requires_raw_output
  implementation
• returns True because the generated provider configures structured output as
  raw JSON
• adds a regression test that generates and imports the schema, then
  instantiates it through from_examples()

The patch is limited to the generator template and its provider-plugin
regression coverage. It restores the documented workflow without changing
runtime APIs.

How Has This Been Tested?

pytest -q -m "not live_api and not requires_pip and not integration"
pyink scripts/create_provider_plugin.py tests/provider_plugin_test.py --check --diff --config pyproject.toml
isort scripts/create_provider_plugin.py tests/provider_plugin_test.py --check-only --diff
pylint --rcfile=tests/.pylintrc tests/provider_plugin_test.py

The local suite passed with 707 tests and 47 subtests. CI also passes
formatting, Python 3.10 through 3.12, and the plugin and Ollama integration
suites.

Checklist:

• I have read and acknowledged Google's Open Source Code of conduct.
• I have read the Contributing page and am covered by the CLA check.
• I have discussed the fix with code owners in the linked issue.
• No documentation change is needed because the provider guide already documents requires_raw_output.
• I added a regression test that exercises the generated schema.
• I followed Google's Python style guidance and ran pylint over the affected test file.

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