feat(openaicompat): forward additional provider options

[ibetitsmike]

Summary

• Add typed OpenAI-compatible provider options for parallel_tool_calls, max_completion_tokens, prompt_cache_key, and extra_body.
• Forward typed options into ChatCompletionNewParams when set.
• Copy extra_body into Chat Completions request extra fields so callers can pass provider-specific request params.
• Add tests for set and unset behavior.
• Fix branch CI compatibility by replacing Go 1.26-only test syntax, allowing previous-response tool outputs to reference calls stored server-side, keeping the module minimum at Go 1.25, and adding toolchain go1.26.3 so govulncheck uses a patched toolchain.
• Update golang.org/x/net to the fixed version reported by govulncheck.

Motivation

Coder Agents settings need typed parity with native OpenAI Chat Completions controls. OpenAI-compatible endpoints also commonly expose provider-specific fields, so extra_body provides a hidden escape hatch without requiring a Fantasy release for every compatible-provider extension.

Mux is working on this on behalf of Mike.

Scope

This PR adds parallel_tool_calls, max_completion_tokens, prompt_cache_key, and extra_body. max_tool_calls is excluded because it is Responses-only and is not available on ChatCompletionNewParams.

Tests

• go test ./...
• govulncheck ./...
• GOTOOLCHAIN=go1.25.10 go test ./providers/openaicompat ./providers/openai ./providertests
• go vet ./providers/openaicompat/...
• gofmt -l providers/openaicompat