Bound HTTP requests in built-in poll callbacks - #1779

Fixes #5.

Subscriptions.read() (the JSON-on-disk path) converts pre-3.02 bindings
that carry 'prefix'+'sub' into the post-3.02 shape with a single 'topic'
key. Subscriptions([...]) built in memory -- the pattern used by
sr_insects' moth_api_consumer.py, flow_api_consumer.py, and the
flow_maint_test.sh harness -- did not. The raw prefix+sub dicts then
hit `topic = b['topic']` in moth/amqp.py:423 and moth/mqtt.py:233 and
raised KeyError: 'topic', which the connect retry loop caught and
spun on until CI killed the job at 20 minutes.

A second KeyError of the same class, 'bindings_to_remove', fired once
the topic issue was patched -- moth/amqp.py:432 iterates
subscription['bindings_to_remove'] directly, but in-memory subs never
populate that key (it's only filled by finalize() when diffing).

Changes to sarracenia/config/subscription.py:

- Extract the conversion into two module-level helpers:
    _broker_proto(broker)      -> 'amqp' | 'mqtt'
    normalize_subscription(s)  -> mutates in place, idempotent
- Subscriptions.__init__ normalizes each passed-in subscription so
  Subscriptions([...]) produces the same shape as Subscriptions.read().
- Subscriptions.add() normalizes new_subscription before merging so
  late additions of pre-3.02 bindings work too.
- read() now delegates to normalize_subscription instead of repeating
  the conversion inline (single source of truth).
- normalize_subscription also defaults bindings_to_remove = [] so the
  downstream moth iterators can't KeyError on in-memory subs.

Tests:

Adds tests/sarracenia/subscription_test.py with 14 cases covering:
- AMQP dot-joining of prefix+sub into topic
- MQTT slash-joining with $share/<queue_name>/ shared-subscription prefix
- Idempotence (running normalize twice produces the same result)
- Leaving a binding that already has 'topic' untouched
- Stripping leftover sub/prefix when 'topic' is also present
- Publisher-side subs without a 'queue' key
- Scalar (non-list) prefix and sub values
- Subscriptions([...]) normalizes on construction (the regression case)
- Subscriptions() / Subscriptions(None) / Subscriptions([]) all empty
- Subscriptions.add() normalizes
- Subscriptions.add() merges a second pre-3.02 binding into an
  existing subscription, with topic computed for both
- bindings_to_remove defaulted to [] by both normalize and __init__

Verification:

- 14/14 new unit tests pass.
- 28/28 sarracenia/config + credentials + subscription tests pass.
- 15/15 sarracenia/moth tests pass (excluding amq1 which needs proton).
- sr_insects/static_flow/moth_api_consumer.py runs cleanly against
  amqps://anonymous@hpfx.collab.science.gc.ca and prints
  "got 5 messages" with no errors in the log.
- sr_insects/static_flow/flow_maint_test.sh EXIT=0, all 5 tests pass:
    test 1 success (queues baseline)
    test 2 success (moth_api_consumer.py consumes 5)
    test 3 success (flow_api_consumer.py consumes 5)
    test 4 success (post-cleanup queue count)
    test 5 success (erase test)
  Previously this script hung at test 3 for the full 20-min CI budget.

…tion

Fix KeyError in Moth when Subscriptions built in memory (issue #5)

Brings the fork's development branch up to upstream as of 25e9335
(Merge pull request MetPX#1691 from MetPX/issue1690).

Preserves fork-only work:
- e5df257 fix: normalize Subscriptions built in memory, default
  bindings_to_remove (fork issue #5) - kept, no upstream conflict.

Drops superseded fork-only work:
- 8c7d7a7 Fix MQTT rx_msg thread-safety: lock callback append,
  remove sleep - resolved by taking upstream's mqtt.py entirely.
  The mutex-based fix has been superseded by my own queue.Queue
  redesign (b7acf4b "Simplify MQTT message reception with
  queue.Queue") that landed upstream via local development.

Restore the working directory after Flow work