[Data Connect] Bug: Swift SDK generates conflicting property/class names

### [REQUIRED] Environment info

**firebase-tools:** 15.1.0

**Platform:** macOS

### [REQUIRED] Test case

Create a Firebase Data Connect mutation with an underscore-prefixed or PascalCase name:

```graphql
# File: dataconnect/app-connector/mutations.gql

mutation _Placeholder @auth(level: NO_ACCESS) {
company_upsert(data: { cik: "_placeholder" })
}
```
**Configure Swift SDK generation in connector.yaml:**

connectorId: app
generate:
swiftSdk:
outputDir: ../apps/ios/MyApp/DataConnect
package: MyAppDataConnect

**[REQUIRED] Steps to reproduce**

1. Create a Data Connect connector with a mutation named _Placeholder (or any PascalCase name like CreateUser)
2. Run firebase dataconnect:sdk:generate
3. Open the generated Swift project in Xcode
4. Attempt to build the project

**[REQUIRED] Expected behavior**

The generated Swift SDK should compile without errors. The SDK generator should ensure that property names use camelCase to avoid conflicts with class names.

**Expected generated code:**
// Property uses camelCase, class uses PascalCase - no conflict
public let placeholderMutation: PlaceholderMutation

**[REQUIRED] Actual behavior**

The generated Swift code produces a compilation error because the property name and class name are identical:

Xcode Error:
```
Use of 'PlaceholderMutation' refers to instance method rather than class 'PlaceholderMutation' in module 'MyAppDataConnect'

Generated code (AppClient.swift):
public class AppConnector {
init(dataConnect: DataConnect) {
// Property and class have the same name - causes Swift conflict
self.PlaceholderMutation = PlaceholderMutation(dataConnect: dataConnect)
}

// MARK: Operations
public let PlaceholderMutation: PlaceholderMutation // ❌ Conflict!
}
```
**Root Cause:**
When the GraphQL operation name starts with uppercase (after removing _ prefix), the SDK generator uses the same casing for both the property name and class name, causing a Swift naming conflict.

| GraphQL Name | Generated Property | Generated Class | Result |
|--------------|---------------------|---------------------|-------------|
| _Placeholder | PlaceholderMutation | PlaceholderMutation | ❌ Conflict |
| CreateUser | CreateUserMutation | CreateUserMutation | ❌ Conflict |
| placeholder | placeholderMutation | PlaceholderMutation | ✅ Works |
| createUser | createUserMutation | CreateUserMutation | ✅ Works |

**Workaround:**
Use camelCase for all GraphQL operation names (e.g., placeholder instead of _Placeholder).

**Suggested Fix:**
The SDK generator should normalize property names to camelCase regardless of the input GraphQL operation name casing.