feat: upgrade mcp-go to v0.43.2 with streamable HTTP transport

[danielheringers]

Summary

• Upgrade github.com/mark3labs/mcp-go from v0.17.0 to v0.43.2
• Migrate MCP tool handlers from request.Params.Arguments to request.GetArguments()
• Add Streamable HTTP as an alternative MCP transport (--transport http-streamable)
• Keep backward compatibility with stdio as the default transport

Changes

• go.mod, go.sum
  • bump mcp-go and update transitive deps
• internal/mcp/handlers_*.go
  • migrate handler argument reads to request.GetArguments()
• internal/mcp/server.go
  • add transport routing (Serve, ServeStdio, ServeStreamableHTTP)
  • add default HTTP bind/path constants
• cmd/vsp/main.go
  • add --transport flag and SAP_TRANSPORT env support
  • normalize and validate transport values (stdio, http-streamable)
• Added tests:
  • internal/mcp/arguments_migration_test.go
  • internal/mcp/server_transport_test.go
  • cmd/vsp/main_test.go
• Docs:
  • README.md, ARCHITECTURE.md, CLAUDE.md

Test plan

• go test -run TestHandlersUseGetArgumentsAPI ./internal/mcp -v
• go test -run "TestServe_|TestServeStreamableHTTP_" ./internal/mcp -v
• go test -run "TestValidateConfig_Transport|TestResolveConfig_Transport|TestRootCmdHasTransportFlag" ./cmd/vsp -v
• go test ./internal/mcp ./cmd/vsp
• go test -cover ./internal/mcp ./cmd/vsp
  • internal/mcp: 9.1%
  • cmd/vsp: 15.1%
• Manual HTTP Streamable smoke test on /mcp
  • initialize: OK
  • notifications/initialized: OK
  • tools/list: OK (non-empty list)

Notes

• go test ./... currently fails in pkg/cache (Example_withSQLite) due CGO/SQLite runtime constraint (CGO_ENABLED=0), which is outside this PR scope.
• Kept GetDependencyZIP compatibility behavior in embedded/deps/embed.go to avoid regressions while related upstream discussion is ongoing.

Fixes #21