fix(server): support extension subscription notifications

[SnowSky1]

Summary

• preserve extension-namespaced keys in subscriptions/listen filters and acknowledgements
• add a generic ServerNotifier.extension() event path filtered by the requested extension key
• cover io.modelcontextprotocol/tasks / notifications/tasks through schema, event-bus, and createMcpHandler tests

Fixes #2569.

Validation

• core schema tests: 3/3
• server focused tests: 25/25
• full pre-push hook: build, lint, and typecheck

Stack note

This draft currently includes #2592 as its first commit because that baseline fix is required for the repository pre-push lint gate. I will rebase this PR onto main after #2592 lands so this diff contains only the extension-subscription commit.

[changeset-bot]

🦋 Changeset detected

Latest commit: 7b63522

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 6 packages

Name	Type
@modelcontextprotocol/server	Patch
@modelcontextprotocol/core	Patch
@modelcontextprotocol/client	Patch
@modelcontextprotocol/core-internal	Patch
@modelcontextprotocol/server-legacy	Patch
@modelcontextprotocol/codemod	Patch

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[pkg-pr-new]

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@modelcontextprotocol/client

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@modelcontextprotocol/codemod

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@modelcontextprotocol/core

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@modelcontextprotocol/server

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@modelcontextprotocol/server-legacy

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@modelcontextprotocol/express

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@modelcontextprotocol/fastify

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@modelcontextprotocol/hono

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@modelcontextprotocol/node

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commit: 7b63522

[SnowSky1]

Closing for the time being: repository permissions prevent me from marking this draft ready for review (the markPullRequestReadyForReview mutation returns FORBIDDEN for my account), and drafts cannot be merged, so I do not want it sitting idle in the review queue. The diff was complete and CI was green at close time; the branch fix/extension-subscription-notifications is kept on my fork. Happy to reopen or resubmit if maintainers want this change or can restore draft-readiness for external contributors.