feat(server): Add hooks.AddOnUnregisterSession functionality

[rwjblue-glean]

Add OnUnregisterSession hook functionality to complement the existing OnRegisterSession hooks, allowing code to run when a client session is being removed from the server. In some cases, the server may want to do additional work when a session has been closed. For example, in the SSE server case where you may end up managing various logs for the duration of the session -- you would want to indicate that the session was finished.

Summary by CodeRabbit

• New Features

  • Added support for hooks that trigger when a client session is unregistered.

• Tests

  • Introduced new tests to verify that session registration and unregistration hooks are called as expected, and to ensure correct behavior when no hooks are present.

[coderabbitai[bot]]

"""

Walkthrough

This change introduces a new hook mechanism for session unregistration events in the server. A new hook type and associated methods are added to the hooks system, enabling registration and invocation of functions when a client session is unregistered. The server's session unregistration logic is updated to trigger these hooks upon session removal. Additionally, tests are added to verify that both registration and unregistration hooks are correctly called, and to ensure that the absence of hooks does not cause errors during session lifecycle operations. Minor updates were made to propagate context parameters in session unregistration calls across several server components.

Changes

Files / Areas Changed	Summary of Changes
server/hooks.go	Added OnUnregisterSessionHookFunc type, new OnUnregisterSession slice field, and methods to add and invoke unregister hooks.
server/server.go	Updated UnregisterSession method to accept context.Context, use atomic session removal, and invoke unregister hooks.
server/server_test.go	Added tests verifying session registration and unregistration hooks, including behavior with nil hooks; simplified error variable declarations in an existing test.
server/sse.go	Updated calls to UnregisterSession to include context.Context parameter; formatting and alignment improvements only.
server/stdio.go	Updated deferred call to UnregisterSession in Listen method to pass context.Context as first argument.

Possibly related issues

• mark3labs/mcp-go#173: Implements explicit hooks and calls for session unregistration events, directly addressing the feature requested in this issue.

Possibly related PRs

• mark3labs/mcp-go#92: Adds context-aware hooks for session registration and complements this PR’s addition of unregister session hooks, both modifying the hooks system and MCPServer session lifecycle handling.
• mark3labs/mcp-go#62: Introduces the initial hooks framework for request lifecycle events; this PR builds on that foundation by adding session unregistration hooks and integrating them into session management.

[!TIP]

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📜 Recent review details

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📥 Commits

Reviewing files that changed from the base of the PR and between e0211e4236c733942c372600bae119f75861f088 and 435622d1b3f1194924350e999c330c50073cd6f4.

⛔ Files ignored due to path filters (1)

• server/internal/gen/hooks.go.tmpl is excluded by !**/gen/**

📒 Files selected for processing (5)

• server/hooks.go (3 hunks)
• server/server.go (1 hunks)
• server/server_test.go (2 hunks)
• server/sse.go (6 hunks)
• server/stdio.go (1 hunks)

🚧 Files skipped from review as they are similar to previous changes (3)

• server/server.go
• server/hooks.go
• server/server_test.go

🔇 Additional comments (6)

server/stdio.go (1)

207-207: Context parameter added correctly to UnregisterSession call

The change correctly adds the context parameter to the UnregisterSession call, enabling context propagation for the new unregister session hooks. This aligns with the PR objective of adding hooks functionality for when sessions end.

server/sse.go (5)

57-66: Code formatting improvements look good

The formatting changes in the SSEServer struct declaration improve readability with consistent indentation.

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70-70: Minor formatting improvement

The addition of a blank line between field groups improves readability.

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164-170: Formatting improvements in SSEServer initialization

The alignment changes in the struct initialization improve code readability and consistency.

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262-262: Context parameter added correctly to UnregisterSession call

The change correctly adds the request context to the UnregisterSession call, which is essential for the new session unregistration hooks functionality. This allows hooks to access the request context when a session is unregistered.

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441-441: Whitespace improvements for better code organization

The addition of blank lines between function definitions improves code readability and organization.

Also applies to: 453-453, 465-465

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