Fix modular pipeline: check required before applying default in get_block_state

[lorenzozanee]

Summary

Fixes #14388

InputParam.template("num_inference_steps", required=True) was silently ignored because get_block_state would substitute the template's default before checking required. This moves the required check before the default fallback.

• Adds regression tests covering missing required inputs (with/without template defaults), satisfied required inputs, and optional defaults.
• Builds on the fix and tests by @yiyixuxu.

AI self-review: READY
Validated: test_modular_pipelines_custom_blocks.py, test_conditional_pipeline_blocks.py, ruff, check_copies, check_dummies, check_support_list.
Upstream-only CI: pr_tests.yml (GPU jobs), pr_tests_gpu.yml, pr_modular_tests.yml, pr_dependency_test.yml, pr_torch_dependency_test.yml, build_documentation.yml, build_pr_documentation.yml, pr_style_bot.yml.

[lorenzozanee]

AI self-review: READY
Validated: test_modular_pipelines_custom_blocks.py (5 new regression + 18 existing, 2 skipped), test_conditional_pipeline_blocks.py (31 passed), ruff (All checks passed), check_copies, check_dummies, check_support_list (all clean), git diff --check (clean).
Upstream-only CI: pr_tests.yml (GPU jobs), pr_tests_gpu.yml, pr_modular_tests.yml, pr_dependency_test.yml, pr_torch_dependency_test.yml, build_documentation.yml, build_pr_documentation.yml, pr_style_bot.yml, claude_review.yml (AI review job).