feat(providers): add opencode_free and kilo_free providers

[Kayphoon]

Summary

Adds two new free-tier provider types for accessing AI models without traditional API keys:

• opencode_free — connects to OpenCode Zen (https://opencode.ai/zen) using ULID session/request headers (x-opencode-session, x-opencode-request), no Authorization header required
• kilo_free — connects to Kilo Gateway (https://api.kilo.ai/api/gateway) using a fixed Bearer token free

Key features

• Both providers auto-create a Default key on provider creation via FixedProviderTemplate reconciliation, so users don't need to manually add keys
• auto_fetch_models is enabled by default for immediate model discovery
• Both support openai:chat API format
• opencode_free model fetch uses a dedicated URL builder to construct /zen/v1/models instead of the default /models path (matching the endpoint's custom_path=/v1 configuration)
• Backend validation whitelist and frontend provider dropdown updated to accept both types

Files changed

• Transport layer: new opencode_free/ and kilo_free/ modules in aether-provider-transport (auth, policy, request, URL builders)
• Pool adapters: new opencode_free.rs and kilo_free.rs in aether-provider-pool (model-only adapters)
• Model fetch: aether-model-fetch strategy selection + opencode_free URL special case
• FixedProviderTemplate: FixedProviderKeyTemplate struct + default key reconciliation logic
• Frontend: provider type dropdown + API format types

Verification

• cargo test -p aether-provider-transport provider_types::tests:: → 16 passed
• cargo test -p aether-gateway handlers::admin::provider::write::provider::template::tests → 3 passed
• cargo test -p aether-model-fetch build_models_fetch_url → 7 passed
• cargo build --release --bin aether-gateway → passed
• Local single-port deployment tested: creating fresh opencode_free/kilo_free providers auto-creates Default keys with auto_fetch_models=true; opencode_free discovers 46 models from upstream