refactor: Restructure SDK into subpackages & implement versioned HTTP transport layers

[stenalpjolly]

Summary

Restructures the Java SDK package layout into domain-specific subpackages (client, transport, auth, tool, exception) instead of a single flat root package, and refactors the HTTP transport layer to delegate version-specific initialization and requests to dedicated subclass routers.

Expectation & Implementation

• Restructured Package Layout:
  • com.google.cloud.mcp (entrypoint classes)
  • com.google.cloud.mcp.client (client implementation classes)
  • com.google.cloud.mcp.transport (transport interfaces and HttpMcpTransport router)
  • com.google.cloud.mcp.transport.vXXXX (version-specific transport subclasses: v20241105, v20250326, v20250618, v20251125)
  • com.google.cloud.mcp.auth (credential handling and resolver classes)
  • com.google.cloud.mcp.tool (tool definition and runner classes)
  • com.google.cloud.mcp.exception (McpException)
• Versioned Transports:
  • Refactored HttpMcpTransport to instantiate and delegate all calls to a version-specific subclass implementation depending on the selected ProtocolVersion.
  • Moved shared fields/methods into BaseMcpTransport to reduce duplication.
• Example & Demo Applications:
  • Updated all imports and configurations inside example/ and demo-applications/cymbal-transit/ to resolve the new SDK packages successfully.

Test cases

• Run all unit tests locally excluding E2E:
  mvn test -Dtest="*Test,!*E2ETest" -Dnet.bytebuddy.experimental=true

Acceptance criteria

• All 129 unit tests pass successfully.
• Code conforms to google-java-format checkstyle standards.
• Examples and Cymbal Transit demo app compile successfully.

Breaking changes

• Yes, this is a breaking change because classes like McpToolboxClientBuilder, ToolDefinition, CredentialsProvider were moved from com.google.cloud.mcp package to domain subpackages. Callers must update their import declarations.