fix: apply tiered pricing for Gemini models via Vertex AI (#8018)

* fix: apply tiered pricing for Gemini models via Vertex AI

- Modified calculateCost method to handle models where cacheReadsPrice is only defined in tiers
- Added comprehensive tests for Vertex AI tiered pricing calculation
- Fixes issue where local cost calculation always showed highest tier rates

Fixes #8017

* Delete src/api/providers/__tests__/vertex-tiered-pricing.spec.ts

---------

Co-authored-by: Roo Code <roomote@roocode.com>
Co-authored-by: Daniel <57051444+daniel-lxs@users.noreply.github.com>

src/api/providers/gemini.ts

@@ -286,10 +286,7 @@ export class GeminiHandler extends BaseProvider implements SingleCompletionHandl
 		outputTokens: number
 		cacheReadTokens?: number
 	}) {
-		if (!info.inputPrice || !info.outputPrice || !info.cacheReadsPrice) {
-			return undefined
-		}
-
+		// For models with tiered pricing, prices might only be defined in tiers
 		let inputPrice = info.inputPrice
 		let outputPrice = info.outputPrice
 		let cacheReadsPrice = info.cacheReadsPrice
@@ -306,6 +303,16 @@ export class GeminiHandler extends BaseProvider implements SingleCompletionHandl
 			}
 		}
 
+		// Check if we have the required prices after considering tiers
+		if (!inputPrice || !outputPrice) {
+			return undefined
+		}
+
+		// cacheReadsPrice is optional - if not defined, treat as 0
+		if (!cacheReadsPrice) {
+			cacheReadsPrice = 0
+		}
+
 		// Subtract the cached input tokens from the total input tokens.
 		const uncachedInputTokens = inputTokens - cacheReadTokens