feat: add opt-in stateless mode for streamable-http transport

[alecron]

Summary

• Add SYSDIG_MCP_STATELESS env var (default false) to enable stateless mode for the streamable-http transport
• When enabled, each HTTP request is self-contained — no initialize handshake or Mcp-Session-Id tracking required
• Required for AWS Bedrock AgentCore compatibility, where invoke-agent-runtime is a single-shot HTTP POST with no session continuity

AgentCore deployment config

SYSDIG_MCP_TRANSPORT=streamable-http
SYSDIG_MCP_STATELESS=true
SYSDIG_MCP_LISTENING_PORT=8000
SYSDIG_MCP_MOUNT_PATH=/mcp

Test plan

• just check passes (fmt, lint, 142/142 specs)
• Manual test: tools/list succeeds without prior initialize when stateless
• Manual test: no Mcp-Session-Id header returned in stateless mode
• Manual test: mcp-remote connects and proxies through stateless server
• Default behavior (SYSDIG_MCP_STATELESS=false) unchanged — not a breaking change

[Copilot]

Pull request overview

Adds an opt-in “stateless” mode for the streamable-http transport to support single-shot HTTP invocation environments (e.g., AWS Bedrock AgentCore), controlled via SYSDIG_MCP_STATELESS.

Changes:

• Add Stateless boolean to configuration loaded from SYSDIG_MCP_STATELESS (default false).
• Thread stateless flag into streamable-http server construction via mcp-go streamable HTTP options.
• Extend handler tests to cover tools/list without prior initialize in stateless mode, and document the env var in README.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
internal/infra/mcp/mcp_handler.go	Adds a stateless parameter and passes the corresponding option to the streamable HTTP server.
cmd/server/main.go	Wires cfg.Stateless into server startup for the streamable-http transport and logs the flag.
internal/config/config.go	Introduces Config.Stateless and loads it from SYSDIG_MCP_STATELESS.
internal/config/config_test.go	Adds assertions for default/override behavior of the new config flag.
internal/infra/mcp/mcp_handler_test.go	Updates existing streamable-http setup for new signature and adds a stateless test case.
README.md	Documents SYSDIG_MCP_STATELESS and its purpose.

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