Report missing Anthropic WIF fields clearly

[yuyua9]

Invalid Anthropic Workload Identity Federation configuration currently raises a raw KeyError when a required field is missing, which makes connection setup failures harder to diagnose.

This PR:

• Validates the four required workload_identity fields before indexing them.
• Raises AnthropicError listing missing or empty fields.
• Adds parameterized regression tests for missing and empty values while preserving optional workspace_id and scope handling.

Validation:

• uv run --project providers/anthropic --frozen pytest providers/anthropic/tests/unit/anthropic/hooks/test_anthropic.py -xvs — 87 passed.
• uv run --project providers/anthropic --frozen ruff format providers/anthropic/src/airflow/providers/anthropic/hooks/anthropic.py providers/anthropic/tests/unit/anthropic/hooks/test_anthropic.py
• uv run --project providers/anthropic --frozen ruff check --fix providers/anthropic/src/airflow/providers/anthropic/hooks/anthropic.py providers/anthropic/tests/unit/anthropic/hooks/test_anthropic.py
• The Breeze provider test command was not run because Breeze is unavailable in the local environment.

Was generative AI tooling used to co-author this PR?

• Yes — Codex (GPT-5)

Generated-by: Codex (GPT-5) following the guidelines

[boring-cyborg]

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