fix(connection): re-raise OSError on initial connect so failures are reported

[plusky]

Problem

Connection.connect()'s OSError branch logged the failure but did not
re-raise when quiet=False:

except OSError:
    if quiet:
        logger.debug("No valid connection to %s:%s (yet)", ...)
    else:
        logger.error("No valid connection to %s:%s", ...)   # falls through

__init__ always calls connect(quiet=False), so an unreachable host
(connection refused / timed out / no route) yielded a Connection with a dead
transport that __init__ returned successfully. Target.connect wraps only
the constructor in its except Exception -> ConnectingTargetFailedMessage
handler, so control then fell through to is_locked() and
parse_system(self.connection) running against the dead transport — crashing
with an opaque low-level error instead of the intended
"connecting to failed: ".

(Auth failures and generic SSHException already re-raise; only the plain
network OSError was swallowed.)

Fix

Re-raise in the non-quiet OSError branch. The quiet=True reconnect path
(host rebooting) still swallows it and lets reconnect()'s is_active() loop
decide when to give up — unchanged.

Tests

• The previously-pinning test_connect_oserror_is_logged_not_raised is updated
  (now ..._is_logged_and_raised) to assert the initial connect raises while
  still logging "No valid connection".
• New test_connect_failure_surfaces_connecting_failed_message (target) asserts
  Target.connect logs ConnectingTargetFailedMessage and re-raises when the
  connection fails.
• The quiet=True swallow test is unaffected and still passes.

Gates: ruff format/check clean, ty clean, pytest green (126 connection+target).

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.43%. Comparing base (1d8dd6b) to head (0725830).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #213   +/-   ##
=======================================
  Coverage   86.43%   86.43%           
=======================================
  Files         159      159           
  Lines        8831     8832    +1     
=======================================
+ Hits         7633     7634    +1     
  Misses       1198     1198           

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