Automatic merge from submit-queue.

[BOT PR] to update dependencies of proxy

This PR will be merged automatically once checks are successful.
```release-note
none
```

Fix implicit-int-float-conversion warning.

**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>

**Description**

This adds end-to-end support for pass through openai /completions. A
follow-up will add observability and close out the feature.


**Related Issues/PRs**

Continues #1231

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

)

**Description**

This completes metrics and tracing and updates docs. So, now
/v1/completions is complete!

**Related Issues/PRs (if applicable)**

Fixes #1231

---------

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