feat(xml): add eager deserialization for buffered payloads

[stobrien89]

Description

Adds eager XML deserialization for buffered payloads while preserving streaming deserialization for reader inputs.

Changes

• Adds XMLDeserializationMode with:
  • AUTO: eagerly parses bytes and incrementally parses BytesReader inputs.
  • EAGER: always materializes the complete XML tree.
  • STREAMING: preserves the existing pull-parser implementation.
• Adds an eager tree-backed deserializer supporting structures, attributes, lists, maps, flattened collections, wrappers, timestamps, and unknown members.
• Adds xml_deserialization_mode to generated Rest XML, AWS Query, and EC2 Query client configurations.
• Reuses the configured XML codec when deserializing modeled AWS Query errors.
• Preserves custom protocol codec settings unless explicitly overridden.
• Updates generated client snapshots, documentation, and changelog metadata.

Performance

Benchmarked on an x86 m7i instance using Python 3.12.14, with 5,000 warmup and 10,000 measured iterations.

XML-decoding cases improved by 28.1–31.4% at p50.

Case	Streaming p50	Eager p50	Change
restXml_CopyObjectOutput_M	70.24 µs	48.57 µs	30.9% faster
awsQuery_GetMetricDataResponse_S	150.23 µs	106.04 µs	29.4% faster
awsQuery_GetMetricDataResponse_M	431.90 µs	296.30 µs	31.4% faster
awsQuery_GetMetricDataResponse_L	4,156.95 µs	2,988.69 µs	28.1% faster

Cases that do not decode an XML document through the payload codec showed benchmark noise:

Case	Streaming p50	Eager p50	Change
restXml_CopyObjectOutput_Baseline	3.11 µs	3.27 µs	5.3% slower
restXml_GetObject_S	14.55 µs	14.86 µs	2.1% slower
restXml_GetObject_M	14.39 µs	14.86 µs	3.3% slower
restXml_GetObject_L	14.65 µs	14.91 µs	1.7% slower

Compatibility

• Existing XMLCodec construction remains supported.
• AUTO preserves incremental parsing for reader inputs.
• STREAMING retains the previous deserialization path.
• Existing exception types and XML behavior are preserved.
• No schema or generated model changes are required.

Validation

• 2,504 Python tests passed.
• All generated protocol test suites passed.
• Full Java build passed.
• Ruff, Pyright, Spotless, and git diff --check passed.

[stobrien89]

Closing in favor of #849