Fix appsec compressed data support for v1 deser

[ajgajg1134]

Motivation

Turns out these fields aren't always just raw json, but can also be base64 encoded gzipped data.

Changes

Check for [ to decide how to deserialize. (The RFC recommends this approach). Also make errors more explicit to make debugging issues easier.

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[github-actions]

CODEOWNERS have been resolved as:

tests/test_the_test/test_deserializer.py                                @DataDog/system-tests-core
utils/proxy/_deserializer.py                                            @DataDog/system-tests-core
utils/proxy/traces/trace_v1.py                                          @DataDog/system-tests-core

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Accept raw JSON objects for appsec schema values

The new decoder only treats payloads as raw JSON when they start with [; every other valid JSON form (for example {...} or "...") is now forced down the base64+gzip path and fails with Invalid base64. This is a behavioral regression from the previous implementation, which fell back to json.loads(payload) for any non-compressed payload, so tracers that emit uncompressed non-array JSON for _dd.appsec.s.* will now fail deserialization.

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[ajgajg1134]

This fine since the RFC requires that uncompressed payloads begin with [