perf(feature-flagging): bound UFC response memory

[leoromanovsky]

Motivation

The client accepted UFC responses without a size limit. A 12 MiB identity response allocated 25,281,544 bytes. A 20 MiB gzip expansion allocated 42,374,176 bytes from 20,405 compressed bytes. A large response could exhaust the application heap.

Changes and Decisions

• Limit compressed response data to 10 MiB.
• Limit decompressed UFC data to 10 MiB. This limit matches the Go implementation.
• Request gzip explicitly and decode it through bounded streams.
• Reject a known oversized response before reading its body.
• Enforce the same limits for chunked responses and incorrect content lengths.
• Do not retry size-limit failures. Keep the last valid configuration and ETag.
• Do not buffer non-200 response bodies.

Validation

The same allocation probe rejected the 12 MiB identity response after 1,769,896 allocated bytes. It rejected the gzip expansion at 10 MiB after 10,607,168 allocated bytes.

[datadog-prod-us1-4]

🎯 Code Coverage (details)
• Patch Coverage: 98.61%
• Overall Coverage: 79.59% (+21.24%)

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