feat: allow session id to be passed externally MCP-369

[nirinchev]

Proposed changes

Allows for clients to send requests with externally-generated session ids. This means that session initialization is skipped which additionally requires that we switch to pure json responses instead of SSE. Sessions are now going to be created on demand with the externally-supplied id and will not be cleaned up until idleTimeoutMS fires.

[coveralls]

Pull Request Test Coverage Report for Build 21407817722

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

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Details

• 94 of 135 (69.63%) changed or added relevant lines in 3 files are covered.
• 77 unchanged lines in 8 files lost coverage.
• Overall coverage increased (+0.6%) to 82.237%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/transports/streamableHttp.ts	85	126	67.46%

Files with Coverage Reduction	New Missed Lines	%
src/resources/common/debug.ts	1	96.63%
src/transports/streamableHttp.ts	1	73.8%
src/tools/atlasLocal/atlasLocalTool.ts	2	51.35%
src/tools/tool.ts	2	86.71%
src/ui/registry/registry.ts	2	94.44%
src/transports/base.ts	14	80.92%
src/common/connectionManager.ts	18	85.26%
src/server.ts	37	77.96%

Totals
Change from base Build 21032021701:	0.6%
Covered Lines:	7474
Relevant Lines:	8979

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💛 - Coveralls

[Copilot]

Pull request overview

This PR introduces support for externally managed sessions in the MCP server's HTTP transport layer. The feature allows external systems (like Amazon Bedrock AgentCore) to manage session lifecycles by providing their own session IDs, while the server maintains backward compatibility with its existing session management.

Changes:

• Added externallyManagedSessions configuration flag to enable/disable external session management
• Modified HTTP transport to accept session IDs via mcp-session-id header and create sessions on-demand
• Added comprehensive integration tests covering both externally managed and internally managed session modes

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
src/common/config/userConfig.ts	Adds externallyManagedSessions boolean configuration flag
src/common/logger.ts	Adds new log ID for session-not-found events
src/transports/streamableHttp.ts	Refactors session handling to support external session IDs with on-demand session creation
tests/integration/transports/streamableHttp.test.ts	Adds comprehensive test coverage for both external and internal session management modes