feat(ai): move API implementations to src/api with lazy wrappers (phase 2)

Stream implementations move from src/providers/ to src/api/, renamed by
API id (anthropic.ts -> anthropic-messages.ts, google.ts ->
google-generative-ai.ts, mistral.ts -> mistral-conversations.ts,
amazon-bedrock.ts -> bedrock-converse-stream.ts). Every module now
exports exactly stream/streamSimple; shared helpers move alongside.

New ProviderStreams dispatch contract in types.ts, lazyApi() wrapper in
api/lazy.ts, and one .lazy.ts wrapper per API. Bedrock's wrapper keeps
the node-only variable-specifier import and setBedrockProviderModule()
(now taking ProviderStreams).

providers/register-builtins.ts deleted; interim until the compat
entrypoint lands, builtin api-registry registration lives in stream.ts
and lazy wrappers are exported from the root barrel. Old per-API lazy
exports (streamAnthropic, ...) are gone; package.json subpaths retarget
to dist/api/.

packages/ai/scripts/generate-models.ts

@@ -8,7 +8,7 @@ import {
 	CLOUDFLARE_AI_GATEWAY_COMPAT_BASE_URL,
 	CLOUDFLARE_AI_GATEWAY_OPENAI_BASE_URL,
 	CLOUDFLARE_WORKERS_AI_BASE_URL,
-} from "../src/providers/cloudflare.ts";
+} from "../src/api/cloudflare.ts";
 import type { AnthropicMessagesCompat, Api, KnownProvider, Model, OpenAICompletionsCompat } from "../src/types.ts";
 
 const __filename = fileURLToPath(import.meta.url);