feat(coding-agent): add prompt cache miss tracking (#6427)

Detect prompt cache misses per turn by comparing each assistant message's
cache reads against the previous request's prompt tokens (core/cache-stats.ts).
Significant misses emit a warning-colored transcript notice at the turn they
occur, noting idle gaps past the cache TTL and model switches when relevant.

/session gained cache statistics: a compact token/cache breakdown with hit
rate, a $-prefixed cost section with per-model cost breakdown, and the
cumulative cost re-billed due to cache misses.

packages/coding-agent/test/suite/regressions/4167-thinking-toggle-pending-tool-render.test.ts

@@ -42,8 +42,8 @@ type RenderSessionContextThis = {
 		getShowImages(): boolean;
 		getImageWidthCells(): number;
 	};
-	sessionManager: { getCwd(): string };
-	session: { retryAttempt: number };
+	sessionManager: { getCwd(): string; getEntries(): SessionEntry[] };
+	session: { retryAttempt: number; modelRegistry: { find(provider: string, modelId: string): undefined } };
 	toolOutputExpanded: boolean;
 	isInitialized: boolean;
 	updateEditorBorderColor(): void;
@@ -71,8 +71,8 @@ function createFakeInteractiveModeThis(): RenderSessionContextThis {
 			getShowImages: () => false,
 			getImageWidthCells: () => 60,
 		},
-		sessionManager: { getCwd: () => process.cwd() },
-		session: { retryAttempt: 0 },
+		sessionManager: { getCwd: () => process.cwd(), getEntries: () => [] },
+		session: { retryAttempt: 0, modelRegistry: { find: () => undefined } },
 		toolOutputExpanded: false,
 		isInitialized: true,
 		updateEditorBorderColor: vi.fn(),