feat(config): support custom HTTP headers (#914)

Integrates the useful custom HTTP header support from #881 onto current main.

- support root, provider-specific, and DEEPSEEK_HTTP_HEADERS overrides
- apply validated extra headers to model API requests while preserving protected Authorization and Content-Type defaults
- document the config shape in README, config.example.toml, and docs/CONFIGURATION.md

Co-authored-by: Desheng <8596814+dst1213@users.noreply.github.com>

config.example.toml

@@ -20,6 +20,9 @@ api_key = "YOUR_DEEPSEEK_API_KEY" # must be non-empty
 base_url = "https://api.deepseek.com"
 # base_url = "https://api.deepseeki.com"         # China users
 # base_url = "https://api.deepseek.com/beta"     # DeepSeek beta features such as strict tool mode
+# Optional custom model request headers for OpenAI-compatible gateways.
+# Authorization and Content-Type are managed by the client and cannot be overridden here.
+# http_headers = { "X-Model-Provider-Id" = "your-model-provider" }
 
 # ─────────────────────────────────────────────────────────────────────────────────
 # Default Models
@@ -161,6 +164,7 @@ max_subagents = 10 # optional (1-20)
 # api_key = "YOUR_DEEPSEEK_API_KEY"
 # base_url = "https://api.deepseek.com"
 # model = "deepseek-v4-pro"
+# http_headers = { "X-Model-Provider-Id" = "your-model-provider" } # optional custom request headers
 
 # NVIDIA NIM-hosted DeepSeek V4 (https://build.nvidia.com)
 [providers.nvidia_nim]