feat(smithy-json): add configurable eager deserialization

[stobrien89]

Description

Adds configurable eager JSON deserialization for buffered payloads while preserving incremental parsing for streaming readers.

Changes

• Adds JSONDeserializationMode with AUTO, EAGER, and STREAMING modes.
• Defaults to AUTO, which eagerly parses bytes and incrementally parses BytesReader inputs.
• Adds an eager deserializer backed by Python's native JSON parser.
• Exposes json_deserialization_mode through generated Rest JSON and AWS JSON client configurations.
• Allows users to explicitly restore the existing streaming behavior.
• Preserves handling for Smithy documents, blobs, timestamps, non-finite floats, nulls, and unknown members.
• Adds runtime, code generation, compatibility, and protocol tests.
• Documents the performance and memory tradeoff between eager and streaming modes.

Performance

Benchmarks used Python 3.12.14 on x86_64 with 5,000 warmup iterations and 10,000 measured iterations per case.

Every measured JSON deserialization case improved.

Summary

The combined values below are the sums of the p50 values for each protocol's cases.

Protocol	Cases	Before p50	After p50	Improvement
Rest JSON 1	5	88.273 µs	70.296 µs	20.4%
AWS JSON 1.0	8	1,965.878 µs	912.633 µs	53.6%
AWS JSON 1.1	8	1,965.691 µs	920.010 µs	53.2%
Overall	21	4,019.842 µs	1,902.939 µs	52.7%

Rest JSON 1

Case	Before p50	After p50	Improvement
restJson1_CopyObjectOutput_Baseline	3.233 µs	3.160 µs	2.3%
restJson1_CopyObjectOutput_M	41.624 µs	24.440 µs	41.3%
restJson1_GetObject_S	14.364 µs	14.185 µs	1.2%
restJson1_GetObject_M	14.523 µs	14.237 µs	2.0%
restJson1_GetObject_L	14.529 µs	14.274 µs	1.8%

AWS JSON 1.0

Case	Before p50	After p50	Improvement
awsJson1_0_GetItemOutput_Baseline	10.946 µs	7.877 µs	28.0%
awsJson1_0_GetItemOutput_S	44.745 µs	21.305 µs	52.4%
awsJson1_0_GetItemOutput_M	132.478 µs	59.910 µs	54.8%
awsJson1_0_GetItemOutput_L	762.325 µs	342.355 µs	55.1%
awsJson1_0_GetItemOutputBinary_S	47.859 µs	23.112 µs	51.7%
awsJson1_0_GetItemOutputBinary_M	140.018 µs	66.401 µs	52.6%
awsJson1_0_GetItemOutputBinary_L	816.823 µs	383.674 µs	53.0%
awsJson1_0_HealthcheckResponse_Example	10.684 µs	7.999 µs	25.1%

AWS JSON 1.1

Case	Before p50	After p50	Improvement
awsJson1_1_GetItemOutput_Baseline	10.653 µs	8.067 µs	24.3%
awsJson1_1_GetItemOutput_S	45.079 µs	21.330 µs	52.7%
awsJson1_1_GetItemOutput_M	131.044 µs	59.105 µs	54.9%
awsJson1_1_GetItemOutput_L	764.908 µs	347.643 µs	54.6%
awsJson1_1_GetItemOutputBinary_S	47.176 µs	22.845 µs	51.6%
awsJson1_1_GetItemOutputBinary_M	140.314 µs	66.233 µs	52.8%
awsJson1_1_GetItemOutputBinary_L	815.806 µs	386.805 µs	52.6%
awsJson1_1_HealthcheckResponse_Example	10.711 µs	7.982 µs	25.5%

Compatibility

• Existing codec APIs remain supported.
• Streaming readers continue using incremental parsing by default.
• Users can explicitly select STREAMING to retain the previous parser for buffered payloads.
• Eager mode temporarily materializes the complete JSON value tree and is intended for buffered payloads where throughput is preferred.
• Serialization behavior is unchanged.

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[stobrien89]

Closing in favor of #850