fix: Raise for missing DeepAttrDict keys instead of inserting them

[razor-x]

What

DeepAttrDict — the type behind custom_metadata on devices, connected accounts, phones, connect webviews, and events, and the fallback for unknown event/action-attempt variants — silently auto-vivified on read: device.custom_metadata.reservaton_id returned {} and wrote the typo'd key into the resource, which then re-serialized back to the API on the next update (SDK audit finding M9, runtime-verified). MIGRATION.md claimed this exact v1 footgun was fixed.

• Reading a missing key now raises like a plain dict: KeyError when indexing, AttributeError for attribute access — matching the typed resource dataclasses and what MIGRATION.md already promises. __getattr__ translates KeyError to AttributeError so hasattr, getattr(..., default), and copy/pickle protocol probes behave normally.
• Nothing is ever inserted on read. Deep-wrapping on write is unchanged; .get(), in, and iteration remain the probes for optional keys.
• MIGRATION.md's misleading "custom_metadata ... not affected" line now states the raising behavior and the .get() probe pattern.

⚠️ Behavior change: code that probed optional custom_metadata keys via bare attribute access and truthy-checked the {} must switch to .get("key") — the pattern the migration guide already prescribes for typed resources.

Testing

Extended test/deep_attr_dict_test.py (previously a single 7-line happy path): missing key raises KeyError / missing attribute raises AttributeError; reads never insert; hasattr/getattr-default/.get/in all behave; nested wrapping intact; end-to-end — a typo'd custom_metadata read on a hydrated device raises and leaves the metadata unmutated; unknown-event fallback fields stay readable.

Revert check: with seam/deep_attr_dict.py reverted to main, the tests fail with DID NOT RAISE AttributeError — the silent-vivification symptom.

Full suite: 192 passed; mypy, pylint (10.00), black clean.

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https://claude.ai/code/session_01Y1RzepycXEYA3LStfjt8cY

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