fix: force HTTP/1.1 for LLM API requests

[efecnc]

Summary

• Forces HTTP/1.1 for all LLM API requests by adding .http1_only() to the shared reqwest client builder
• Fixes "error decoding response body" failures with reasoning models (deepseek-v4-pro, deepseek-reasoner)

Problem

reqwest with rustls-tls negotiates HTTP/2 via ALPN. DeepSeek's API (behind CloudFront) has HTTP/2 body-framing issues on long-running responses — when a reasoning model takes >30s to respond, the HTTP/2 stream frame boundaries can corrupt or truncate the response body, causing reqwest's .text().await to fail with "error decoding response body."

This error persisted even after PR #20 increased the timeout to 600s, confirming it's not a timeout issue but an HTTP/2 protocol-level problem.

Why HTTP/1.1

• Single-request LLM API calls have zero performance benefit from HTTP/2 multiplexing
• HTTP/1.1 has decades of battle-tested reliability for simple request-response patterns
• Eliminates an entire class of hard-to-debug protocol errors
• No impact on other features (web_search, web_fetch use separate clients)

Test plan

• Verify deepseek-v4-pro completes requests without "error decoding response body"
• Verify Gemini, OpenAI, OpenRouter providers still work normally
• Confirm no regression for Anthropic provider (uses separate client path)

[gemini-code-assist]

Code Review

This pull request modifies the global reqwest client builder in src/utils.rs to force HTTP/1.1. The review feedback identifies that this change impacts the Anthropic provider, contrary to the PR's initial assumptions, and suggests adding a code comment to document the rationale for this protocol restriction to avoid future regressions.

[gemini-code-assist]

The PR description's assumption that the Anthropic provider uses a separate client path and is unaffected is incorrect; AnthropicProvider in src/provider.rs (line 92) calls this build_reqwest_client() function. Consequently, this change forces HTTP/1.1 for Anthropic as well, which contradicts the stated test plan and indicates that this provider may not have been verified with the protocol change.

[efecnc]

REJECTED — Over-engineering

This PR forces HTTP/1.1 for all providers to fix a problem that only affects DeepSeek behind CloudFront.

Problems:

1. Overly broad: Disables HTTP/2 for OpenAI, Gemini, Anthropic, OpenRouter when only DeepSeek needs the fix
2. Anthropic is affected (contrary to PR description): AnthropicProvider::new() in src/provider.rs:92 calls build_reqwest_client()
3. Better fix: Apply .http1_only() only to the DeepSeek client path, or make it configurable per-provider

Recommendation:

Close this PR. The correct approach is provider-specific — add .http1_only() only where it's actually needed.

[efecnc]

Reopened with provider-specific fix

The original concern (close comment below) was that .http1_only() was applied globally via build_reqwest_client(), affecting all providers. Fixed in commit 1849e8e:

Changes

• Removed .http1_only() from build_reqwest_client() — OpenAI, Gemini, Anthropic, OpenRouter now use HTTP/2 normally
• Added build_reqwest_client_http1() — separate builder that forces HTTP/1.1
• LLMClient::new_openai_compatible() — stays HTTP/2-capable (default for all OpenAI-compatible providers)
• LLMClient::new_openai_compatible_http1() — forces HTTP/1.1
• create_provider() — checks provider_name for "deepseek" (case-insensitive) and routes only DeepSeek providers to the HTTP/1.1 client

What changed from the original PR

Before (over-engineered)	After (targeted)
.http1_only() on shared build_reqwest_client()	.http1_only() only for DeepSeek
All providers forced to HTTP/1.1	Only DeepSeek gets HTTP/1.1
Anthropic affected (contrary to PR claims)	Anthropic unaffected (uses default build_reqwest_client())

Files changed

• src/utils.rs: split build_reqwest_client() → build_reqwest_client() + build_reqwest_client_http1(); added new_openai_compatible_http1()
• src/provider.rs: create_provider() routes DeepSeek → HTTP/1.1 client

Verification

• ✅ cargo check passes
• ✅ cargo test --lib utils::tests passes (4/4)