feat(plugin): add session request hook

[rekram1-node]

Summary

• expose session.request on the Effect and Promise plugin boundaries
• allow plugins to mutate final outgoing native LLM URLs, HTTP headers, and serialized request bodies
• invoke the hook after native request serialization and authentication
• pass the callback as process-local LLMClient execution options rather than storing runtime behavior in the Schema-backed LLMRequest

Scope

• URL, headers, and body are mutable; method is readonly
• the hook currently applies only to native @opencode-ai/ai HTTP transports
• AI SDK providers continue to use the existing aisdk.sdk and aisdk.language integration hooks
• native WebSocket messages are not represented as HTTP requests by this hook

Validation

• bun typecheck in packages/ai, packages/core, and packages/plugin
• bun test test/compile.test.ts in packages/ai
• bun test test/aisdk.test.ts test/plugin-hooks.test.ts in packages/core

[rekram1-node]

AI SDK support is intentionally deferred from this PR. An initial implementation used AsyncLocalStorage to bridge per-session callbacks into cached AI SDK providers, but we removed it to keep this hook explicit and native-only. We may revisit AI SDK final-request interception later with a dedicated design around its existing aisdk.sdk / aisdk.language hooks or request-bound provider factories.