fix(core): preserve provider metadata namespaces

[opencode-agent]

Summary

• preserve complete provider metadata instead of indexing it by the catalog provider ID
• merge reasoning metadata across start, delta, and end events by namespace
• replay namespaced metadata unchanged through native and AI SDK routes
• retain compatibility with existing flattened session state

Why

The OpenCode provider hosts models backed by multiple API formats while retaining providerID: "opencode". Reasoning metadata is emitted under the underlying namespace, for example anthropic.signature, so indexing it by opencode drops the continuation state.

Verification

• cd packages/core && bun test test/session-runner-tool-events.test.ts test/session-runner-message.test.ts test/aisdk.test.ts (25 pass)
• bun typecheck / push hook attempted, but this host exhausted its OS thread limit in unrelated workspace packages (newosproc, errno=11)

[opencode-agent]

Audit note: this overlaps #35812, which preserves content-level metadata during AI SDK lowering. Please clarify whether this PR builds on #35812 or supersedes it, and consolidate the merge path before either lands.