Fix two plugin-loading bugs: labconfig defaults read as plugin flags, and a setup retry that re-runs side effects

[ispielma]

Two independent plugin bugs found while Claude audite labscript-utils and its
callers. Separate commits, both reproduced against master first.

1. Labconfig [DEFAULT] keys are read as plugin enable flags

ConfigParser sections inherit [DEFAULT], so
exp_config.items('BLACS/plugins') returns the section's own options plus
every labconfig default. Against a typical labconfig:

line 90 sees: ['apparatus_name', 'userlib', 'labscript_suite', 'connection_table']

A plugin directory whose name collides with a [DEFAULT] key therefore never
had an enable flag written into the section, and getboolean() then read the
default's value instead:

ValueError: Not a boolean: /home/u/labscript-suite/userlib

That call is unguarded and the whole block runs at module scope, so
import blacs.plugins raises and BLACS does not start at all, rather than
skipping a single plugin.

The fix writes a real flag into the section when the inherited value is not a
boolean — a section option shadows [DEFAULT], so the plugin resolves from
then on — and swaps the list-membership test for has_option(), which asks the
question directly. An installation that legitimately sets the flag in both
places is unaffected, because the section value already wins:

collision only in [DEFAULT]   before: ValueError   after: starts, plugin disabled
set in both places            before: enabled      after: enabled

2. plugin_setup_complete retry re-runs side effects

The old-API fallback is driven by catching Exception from the call itself, so
any failure inside a correctly-written plugin_setup_complete(data) triggers a
second invocation.

For a plugin whose hook accepts both arities:

def plugin_setup_complete(self, data=None)

the retry re-enters the body, so whatever the first, partially completed call
had already done — started a thread, registered a listener, opened a
connection — happens a second time:

plugin_setup_complete(self, data=None)    before: side effect ran 2x
                                           after: side effect ran 1x

Plugins with a strict arity were never double-run, because the retry failed on
argument count before reaching the body, which is why this has been easy to
miss. Note that labscript_utils.plugins.BasePlugin declares exactly the
optional-argument signature, so this shape is not hypothetical.

The fix binds the signature first and calls once with the arguments it
accepts. Old no-argument plugins still work and still get the deprecation
warning; plugins with no introspectable signature fall back to the current
API. A genuine error now produces one clear log entry instead of a misleading
"Trying again with old call signature" followed by an arity error.

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2 files changed, 35 insertions(+), 10 deletions(-)