fix(core): close session after cancelled initialization

[hsusul]

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🛠️ Fixes #744

Summary

Ensure _McpHttpTransportBase.close() still closes its internally managed
aiohttp.ClientSession when the cached initialization task was cancelled.

Problem

asyncio.CancelledError inherits directly from BaseException, so the existing
except Exception around the initialization task did not handle cancellation.
Calling close() after a cancelled initialization therefore propagated
CancelledError before reaching session cleanup, leaving the managed session
open.

Minimal reproduction

1. Create a core MCP transport that owns its aiohttp.ClientSession.
2. Assign and cancel its cached _init_task.
3. Await the cancelled task, then call await transport.close().

Before this change, close() raises asyncio.CancelledError and
transport._session.closed remains False. After this change, explicit cleanup
completes and the managed session is closed.

Implementation

• Handle asyncio.CancelledError alongside ordinary initialization exceptions
  during explicit transport cleanup.
• Add a regression test proving a cancelled initialization no longer prevents
  the managed session from closing.
• Add adjacent coverage preserving the existing ordinary-initialization-error
  cleanup behavior.

The change does not alter initialization or request-time cancellation behavior,
and externally managed sessions remain unaffected.

Validation

• python -m pytest tests/mcp_transport/test_base.py::TestMcpHttpTransportBase::test_close_managed_session_after_cancelled_initialization -v
  • Before the fix: failed from transport.close() with
    asyncio.exceptions.CancelledError.
  • After the fix: 1 passed.
• Test sensitivity: temporarily reverting only the production clause made the
  regression fail again; restoring it made the test pass.
• python -m pytest tests/mcp_transport/test_base.py -q — 39 passed.
• python -m pytest tests -q --ignore=tests/test_e2e.py --ignore=tests/test_sync_e2e.py --ignore=tests/test_e2e_mcp.py --ignore=tests/conformance --cov=src/toolbox_core --cov-report=term --cov-fail-under=90
  — 482 passed, total coverage 91.80%.
• python -m pytest tests/unit -q in toolbox-adk — 59 passed.
• python -m pytest tests -k 'not e2e' -q in toolbox-langchain —
  60 passed, 64 deselected.
• python -m pytest tests -k 'not e2e' -q in toolbox-llamaindex —
  61 passed, 64 deselected.
• black --check . — passed; 44 files would be left unchanged.
• isort --check . — passed.
• MYPYPATH='./src' mypy --install-types --non-interactive --cache-dir=<temporary-cache> -p toolbox_core
  — Success: no issues found in 29 source files.
• git diff upstream/main...HEAD --check — passed.

Live E2E and conformance suites were not run locally because they require
external Toolbox/GCP infrastructure. Fork CI remains subject to the
repository's maintainer-triggered workflow.

Compatibility and risk

The runtime change is limited to explicit cleanup after an initialization task
has already been cancelled. Ordinary initialization errors, successful
initialization, and unmanaged-session cleanup retain their existing behavior.

Non-goals and related work

• This does not change request-time cancellation propagation or the shared
  initialization behavior addressed independently by fix(core): isolate shared initialization cancellation #745. That PR explicitly
  lists changing close() cancellation semantics as a non-goal and modifies a
  different production hunk.
• This does not retry initialization, change protocol negotiation, or alter
  public APIs.
• This is independent of fix(core): allow load_toolset strict success when all params are used #735, fix(core): await coroutines returned by callable token getters and bound params #736, and fix(adk): support map additionalProperties and bind_param in ToolboxTool #737 and does not touch their files or
  behaviors.

AI assistance

AI assistance was used for repository inspection, implementation, testing, and
drafting this pull request. The contributor reviewed the final diff and
validation evidence.

[anubhav756]

/gcbrun