fix(fetch): implement Request.text(), json(), arrayBuffer() methods

[akapug]

Summary

Implements the missing body-reading methods on FetchRequest interface:

• text(): Returns body as string (UTF-8)
• json(): Parses body as JSON
• arrayBuffer(): Returns body as byte array

Changes

FetchRequest.kt (Interface)

• Added interface method declarations for text(), json(), arrayBuffer()

FetchRequestIntrinsic.kt (Implementation)

• Added rawBodyBytes constructor parameter for storing request body
• Implemented readBodyBytes() helper method
• Added text(), json(), arrayBuffer() implementations
• Added ProxyExecutable wrappers in getMember() for GraalVM interop
• Modified forRequest() factories to capture raw bytes

JsServerRequestExecutionInputs.kt (Server integration)

• Added cachedBodyBytes field with lazy body reading
• Implemented text(), json(), arrayBuffer() methods

Notes

• Implementation is synchronous (returns values directly, not Promises)
• A full async Promise implementation would need additional GraalVM integration
• This unblocks basic MCP server and REST API use cases that need POST body parsing

Testing

Build verified: ./gradlew :packages:graalvm:compileKotlin passes

Fixes #1805