feat: session list API (#23202)

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_Disclaimer:_ _initially_ _produced_ _by_ _Claude_ _Opus_ _4\.6,_ _heavily_ _modified_ _and_ _reviewed_ _by_ _me._

Closes https://github.com/coder/internal/issues/1360

Adds a new `/api/v2/aibridge/sessions` API which returns "sessions".

Sessions, as defined in the [RFC](https://www.notion.so/coderhq/AI-Bridge-Sessions-Threads-2ccd579be59280f28021d3baf7472fbe?source=copy_link), are a set of interceptions logically grouped by a session key issued by the client.  
The API design for this endpoint was done in [this doc](https://github.com/coder/internal/issues/1360).

If the client has not provided a session ID, we will revert to the thread root ID, and if that's not present we use the interception's own ID (i.e. a session of a single interception - which is effectively what we show currently in our `/api/v2/aibridge/interceptions` API).

The SQL query looks gnarly but it's relatively simple, and seems to perform well (~200ms) even when I import dogfood's `aibridge_*` tables into my workspace. If we need to improve performance on this later we can investigate materialized views, perhaps, but for now I don't think it's warranted.

---

_The PR looks large but it's got a lot of generated code; the actual changes aren't huge._

site/src/api/typesGenerated.ts

@@ -75,6 +75,12 @@ export interface AIBridgeListInterceptionsResponse {
 	readonly results: readonly AIBridgeInterception[];
 }
 
+// From codersdk/aibridge.go
+export interface AIBridgeListSessionsResponse {
+	readonly count: number;
+	readonly sessions: readonly AIBridgeSession[];
+}
+
 // From codersdk/deployment.go
 export interface AIBridgeOpenAIConfig {
 	readonly base_url: string;
@@ -95,6 +101,28 @@ export interface AIBridgeProxyConfig {
 	readonly allowed_private_cidrs: string;
 }
 
+// From codersdk/aibridge.go
+export interface AIBridgeSession {
+	readonly id: string;
+	readonly initiator: MinimalUser;
+	readonly providers: readonly string[];
+	readonly models: readonly string[];
+	readonly client: string | null;
+	// empty interface{} type, falling back to unknown
+	readonly metadata: Record<string, unknown>;
+	readonly started_at: string;
+	readonly ended_at?: string;
+	readonly threads: number;
+	readonly token_usage_summary: AIBridgeSessionTokenUsageSummary;
+	readonly last_prompt?: string;
+}
+
+// From codersdk/aibridge.go
+export interface AIBridgeSessionTokenUsageSummary {
+	readonly input_tokens: number;
+	readonly output_tokens: number;
+}
+
 // From codersdk/aibridge.go
 export interface AIBridgeTokenUsage {
 	readonly id: string;