fix(ssh): replace private _username with public get_extra_info API

[xingyug]

Summary

Replace all usages of asyncssh's private conn._username attribute with the public conn.get_extra_info("username") API.

Motivation

The codebase accesses SSHClientConnection._username in 5 places for audit logging and error messages. This is a private attribute (prefixed with _) that is not part of asyncssh's public API contract. While it works today, it could break on any future asyncssh release without notice.

asyncssh provides get_extra_info("username") as the documented public API for retrieving the authenticated username. It returns the same value — verified to be the exact same object (is identity) in testing.

Changes

src/linux_mcp_server/connection/ssh.py (5 replacements):

• Connection pool reuse audit log (L122)
• New connection audit log (L160)
• Command timeout error message (L244)
• get_remote_bin_path() SSH error message (L325)
• get_remote_bin_path() not-found error message (L330)

Tests (3 files, mock pattern updated):

# Before: setting private attribute directly
Mock(SSHClientConnection, _username="testuser")

# After: mocking the public API
mock = Mock(SSHClientConnection)
mock.get_extra_info.return_value = "testuser"

Verification

• make verify passes (ruff ✅, pyright ✅, 487 tests ✅)
• Manually verified get_extra_info("username") returns identical results to _username across multiple scenarios (explicit username, SSH config inferred username)

[github-actions]

For team members: test commit 145428d in internal GitLab

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Flag	Coverage Δ
unittests	96.47% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/linux_mcp_server/connection/ssh.py	93.44% <100.00%> (ø)
tests/connection/ssh/test_connection_manager.py	100.00% <100.00%> (ø)
tests/connection/ssh/test_get_bin_path.py	100.00% <100.00%> (ø)
tests/connection/ssh/test_timeout.py	100.00% <100.00%> (ø)

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