fix(bedrock): drop metrics-only invocation trailer from stream decoder

[camposvinicius]

Problem

Follow-up to #1682. That PR fixed typed-event routing by preserving each chunk's real event type, but the amazon-bedrock-invocationMetrics trailer is still forwarded as event="completion". The trailer carries neither a type nor a completion field, so on a Messages stream the "completion" branch in _streaming.py::__stream__ constructs it against the RawMessageStreamEvent union with no discriminator. The union falls back to its first member and yields a contract-violating RawMessageStartEvent(message=None), which leaks to the consumer as an extra stream event.

Any consumer that trusts the type annotations (for example event.message.usage, as in the original report via pydantic-ai's streaming path on AsyncAnthropicBedrock) then crashes with AttributeError: 'NoneType' object has no attribute 'usage'. This is the same failure reported in #1647, still reproducible on main.

Reproduction (end to end, offline)

Feeding a complete Messages stream plus the metrics trailer through the real AnthropicBedrock client (binary eventstream frames, decoder, raw messages.create(stream=True)):

BEFORE  total events: 8 | last = RawMessageStartEvent  type=None  message=None
        event.message.usage -> AttributeError: 'NoneType' object has no attribute 'usage'

AFTER   total events: 7 | stream is clean, no leaked event

Fix

In lib/bedrock/_stream_decoder.py (hand-maintained, not Stainless-generated), _chunk_bytes_to_sse now drops the metrics-only trailer instead of forwarding it:

• typed Messages events keep their real event type (unchanged)
• legacy text-completion chunks (which carry a completion field) still route as event="completion" (unchanged)
• a legacy completion chunk that also carries the metrics trailer is kept (unchanged)
• a chunk with no type and no completion field, the metrics-only trailer, is dropped so it never reaches the stream-event union

Tests

Adds two regression tests to tests/lib/test_bedrock.py:

• test_chunk_bytes_to_sse_drops_metrics_only_trailer asserts the decoder returns None for the trailer
• test_metrics_trailer_would_violate_stream_event_contract documents the union fallback the drop protects against

Full tests/lib/test_bedrock.py passes, and ruff and pyright are clean on the changed files.

Fixes #1647.