fix(ai): stable metadata.user_id per session for Anthropic OAuth

Anthropic counts sessions by metadata.user_id. Without this fix, OMP
generated fresh random entropy on every API request, inflating the
session count and preventing backend attribution to the authenticated
account.

Changes:

packages/ai:
- resolveAnthropicMetadataUserId() now accepts JSON-format user_id
  matching real Claude Code's getAPIMetadata shape
  ({ session_id, account_uuid, ... }). Previously only the legacy
  cloaking format was accepted on OAuth, causing stable caller-supplied
  values to be silently discarded.
- AnthropicOAuthFlow.exchangeToken() and refreshAnthropicToken() now
  populate OAuthCredentials.{accountId, email} from the token response
  account block, removing the need for a separate /api/oauth/profile
  round-trip.
- AuthStorage.getOAuthAccountId(provider, sessionId) returns the OAuth
  accountId for the session-sticky credential, used to build
  account_uuid in metadata.user_id. Guards against misattribution for
  API-key, runtime-override, env-key, and fallback-resolver paths that
  do not record a session credential.

packages/agent:
- Agent.metadataForProvider(provider) resolves request metadata for
  the given provider via the installed resolver, or returns the static
  metadata value. The plain metadata getter now returns only the static
  value; provider-aware resolution is explicit.
- Agent.setMetadataResolver(fn) installs a (provider: string) resolver
  evaluated per LLM request in agent-loop, after getApiKey records the
  session-sticky credential, so account_uuid reflects the credential
  actually used.
- AgentLoopConfig.metadataResolver is called with config.model.provider
  after getApiKey, overriding the static metadata field.

packages/coding-agent:
- AgentSession.#syncAgentSessionId installs a metadata resolver that
  builds { user_id: JSON.stringify({ session_id, account_uuid? }) },
  matching the Anthropic session attribution format. account_uuid is
  only included for provider="anthropic" to avoid leaking the OAuth
  identity to third-party Anthropic-format-compatible providers.
- sessionId getter prefers providerSessionId when supplied via
  AgentSessionConfig so all API paths (getApiKey, direct calls,
  metadata resolver) share the same provider-facing session ID.
- prepareSimpleStreamOptions stamps session metadata on direct calls
  (runEphemeralTurn, compaction, branch summary, title generation) so
  they share the same session bucket as Agent.prompt requests.
- generateBranchSummary and generateSessionTitle accept a
  (provider: string) metadata resolver evaluated after their own
  getApiKey call for correct credential attribution.

packages/coding-agent/src/modes/controllers/input-controller.ts

@@ -362,7 +362,14 @@ export class InputController {
 			const hasUserMessages = this.ctx.session.messages.some((m: AgentMessage) => m.role === "user");
 			if (!hasUserMessages && !this.ctx.sessionManager.getSessionName() && !$env.PI_NO_TITLE) {
 				const registry = this.ctx.session.modelRegistry;
-				generateSessionTitle(text, registry, this.ctx.settings, this.ctx.session.sessionId, this.ctx.session.model)
+				generateSessionTitle(
+					text,
+					registry,
+					this.ctx.settings,
+					this.ctx.session.sessionId,
+					this.ctx.session.model,
+					provider => this.ctx.session.agent.metadataForProvider(provider),
+				)
 					.then(async title => {
 						if (title) {
 							const applied = await this.ctx.sessionManager.setSessionName(title, "auto");