[Vertex AI] `credentials-uri` property is ignored when transport is set to 'grpc'

Bug description
I am using spring-ai-starter-model-vertex-ai-gemini to analyze audio files using the Gemini model.
I encountered an authentication issue specifically when switching the transport mode to gRPC.

• Scenario A (REST - Working): When spring.ai.vertex.ai.gemini.transport is set to rest (or left default), the application successfully authenticates using the JSON key file defined in spring.ai.vertex.ai.gemini.credentials-uri.
• Scenario B (gRPC - Failing): When I explicitly set spring.ai.vertex.ai.gemini.transport=GRPC (to handle larger payloads/timeouts), the application fails with UNAUTHENTICATED. It appears that the credentials-uri property is ignored or not correctly propagated to the gRPC client builder.
• Workaround: Setting the GOOGLE_APPLICATION_CREDENTIALS environment variable resolves the issue for gRPC mode. This confirms that the key file is valid, but the Spring property mapping for credentials-uri is broken when using the gRPC transport.

Environment

• Spring AI version: 1.1.2
• Spring Boot version: 3.4.4
• Java version: 21
• Kotlin version: 1.9.25
• OS: macOS Tahoe 26.2

Steps to reproduce

1. Configure build.gradle.kts with Spring AI 1.1.2 and Vertex AI Gemini starter.
2. Set application.properties to use gRPC (transport=GRPC) and a file-based credential URI (credentials-uri=file:...).
3. Attempt to call chatModel.call().

Minimal Complete Reproducible example

build.gradle.kts

plugins {
    id("org.springframework.boot") version "3.4.4"
    kotlin("jvm") version "1.9.25"
}

extra["springAiVersion"] = "1.1.2"

dependencies {
    implementation("org.springframework.ai:spring-ai-starter-model-vertex-ai-gemini")
    implementation(platform("org.springframework.ai:spring-ai-bom:${property("springAiVersion")}"))
    implementation("org.springframework.boot:spring-boot-starter-web")
    implementation("com.fasterxml.jackson.module:jackson-module-kotlin")
}

application.properties

# Vertex AI Gemini Configuration
spring.ai.vertex.ai.gemini.project-id=my-project-id
spring.ai.vertex.ai.gemini.location=us-central1
spring.ai.vertex.ai.gemini.chat.options.model=gemini-2.5-flash

# [ISSUE HERE] This credential is ignored when transport is GRPC
spring.ai.vertex.ai.gemini.credentials-uri=file:src/main/resources/service-account-key.json

# If this is set to GRPC, authentication fails (UNAUTHENTICATED).
# If commented out (REST), it works.
spring.ai.vertex.ai.gemini.transport=GRPC

Controller (Kotlin)

@RestController
class TestController(private val chatModel: VertexAiGeminiChatModel) {
    
    @GetMapping("/test")
    fun test(): String {
        // This call fails with UNAUTHENTICATED when transport=grpc
        // unless GOOGLE_APPLICATION_CREDENTIALS env var is set.
        return chatModel.call("Hello World")
    }
}

Stack Trace

io.grpc.StatusRuntimeException: UNAUTHENTICATED: Request had invalid authentication credentials. Expected OAuth 2 access token, login cookie or other valid authentication credential.
...
Caused by: java.net.SocketTimeoutException: Read timed out

Start at the VertexAiGeminiChatModel configuration entry point and trace how credentials-uri is passed when transport is set to GRPC. Reproduce the issue with the supplied application.properties and service-account key, then verify that the gRPC call authenticates without GOOGLE_APPLICATION_CREDENTIALS.