feat: add http_headers support to LlmProvider for custom upstream headers

[octo-patch]

Fixes #592

Problem

LlmProvider supports custom endpoint and access_key but has no way to pass custom HTTP request headers to the upstream provider. Some providers or API gateways require non-standard headers for authentication (e.g., X-API-Key, X-Org-ID) or telemetry purposes that cannot be expressed through the existing access_key field.

Solution

Added an optional http_headers field to LlmProvider that accepts a key-value map of HTTP headers to be injected into every upstream request for that provider:

model_providers:
  - model: provider/model-name
    access_key: $API_KEY
    http_headers:
      X-Custom-Header: "custom-value"
      X-API-Version: "v1"

Changes:

• crates/common/src/configuration.rs: Added http_headers: Option<HashMap<String, String>> field to LlmProvider struct
• crates/common/src/llm_providers.rs: Updated create_test_provider test helper to include the new field
• crates/llm_gateway/src/stream_context.rs: Added set_custom_provider_headers() method that applies configured headers after auth headers are set; called from on_http_request_headers
• config/plano_config_schema.yaml: Added http_headers to the schema for both model_providers and the deprecated llm_providers sections

Custom headers are applied after provider-specific auth headers (Authorization, x-api-key), so they can supplement but not accidentally override authentication.

Testing

The change is backward compatible — the new field is Option<...> and defaults to None, so existing configs are unaffected. The schema addition ensures validation passes for configs using the new field.