Automatic merge from submit-queue.

Check quota cache hit

**What this PR does / why we need it**: When Check() is enabled, Mixer filter sends attributes check.cache_hit and quota.cache_hit in Report() call. When Check() is disabled, Mixer filter does not send these attributes.

**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes envoyproxy#1181 

**Special notes for your reviewer**:

**Release note**:

```release-note
NONE
```

Description: This PR moves all of the library/cc and library/python C++ methods from snake_case to camelCase per Envoy coding guidelines.

Risk Level: Low

Testing: bazel test //library/python/integration:test_send_headers

Docs Changes: N/A

Release Notes: N/A

Signed-off-by: Cerek Hillen <chillen@lyft.com>
Signed-off-by: JP Simard <jp@jpsim.com>

Description: This PR moves all of the library/cc and library/python C++ methods from snake_case to camelCase per Envoy coding guidelines.

Risk Level: Low

Testing: bazel test //library/python/integration:test_send_headers

Docs Changes: N/A

Release Notes: N/A

Signed-off-by: Cerek Hillen <chillen@lyft.com>
Signed-off-by: JP Simard <jp@jpsim.com>

**Description**

This PR adds OpenAI's legacy `/completions` endpoint support to Envoy AI
Gateway, enabling compatibility with models like `babbage-002` and
`gpt-3.5-turbo-instruct`. Ideal for cost-optimized or fine-tuned setups
(e.g., vLLM/llama.cpp) needing suffix params or token prompts.

Refactored unions (`ContentUnion`, `EmbeddingRequestInput`,
`PromptUnion`) with custom unmarshaling for **1.6x–10x speedups**,
**60%–95% less memory**, and **52%–93% fewer allocations** vs.
openai-go. Gains are critical for batch/token-heavy workloads.

*Key Changes*
- New types: `CompletionRequest`, `CompletionResponse`, `PromptUnion`.
- Model constants for legacy testing.
- Unified `Usage` across APIs.
- New Benchmarks!

**Related Issues/PRs**

Starts #1231 

---------

Signed-off-by: Adrian Cole <adrian@tetrate.io>