Refactor Mux subscription lifecycle with ref counting

[KCarretto]

This PR addresses potential race conditions and inefficient subscription management in the Mux component.

Changes:

• Added acquireSubscription in tavern/internal/portals/mux/mux_pubsub.go: Centralizes subscription acquisition logic, including cache checking, provisioning, connection, race handling, and reference counting.
• Updated CreatePortal in tavern/internal/portals/mux/mux_create.go: Now uses acquireSubscription instead of blindly overwriting existing subscriptions. This prevents premature closure if a subscription is already active (e.g. from a race or collision).
• Updated OpenPortal in tavern/internal/portals/mux/mux_open.go: Refactored to use acquireSubscription for consistency and code reuse.
• Added TestMux_CreatePortal_RefCounting in tavern/internal/portals/mux/mux_test.go: Verifies that CreatePortal correctly initializes reference counts and that teardown respects them (subscription remains active if refs > 0).

This ensures that local subscriptions are robustly managed and shared where appropriate.

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PR created automatically by Jules for task 13481624774023790476 started by @KCarretto

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[github-actions]

Summary

Tests 📝	Passed ✅	Failed ❌	Skipped ⏭️	Other ❓	Flaky 🍂	Duration ⏱️
2527    ±0	2527    ±0	0    ±0	0    ±0	0    ±0	0    ±0	1ms    ±0

Previous Results

Build 🏗️	Result 🧪	Tests 📝	Passed ✅	Failed ❌	Skipped ⏭️	Other ❓	Flaky 🍂	Duration ⏱️
#717	✅	2527	2527	0	0	0	0	18.7s

Insights

Average Tests per Run	Total Flaky Tests	Total Failed	Slowest Test (p95)
2527	0	0	5.3s

Slowest Tests

Test 📝	Results 📊	Duration (avg) ⏱️	Duration (p95) ⏱️
eldritch-libsys: std::dll_inject_impl::tests::test_dll_inject_simple	1	5.3s	5.3s
eldritch-stdlib-tests: tests::test_all_tomes	3	5.1s	5.1s
eldritch-stdlib-tests: tests::test_all_tomes	3	5.1s	5.1s
eldritch-stdlib-tests: tests::test_all_tomes	3	5.1s	5.1s
imix::bin/imix: tests::task_tests::test_task_streaming_error	3	3.0s	3.0s
imix::bin/imix: tests::task_tests::test_task_streaming_error	3	3.0s	3.0s
imix::bin/imix: tests::task_tests::test_task_streaming_error	3	3.0s	3.0s
imix::bin/imix: tests::task_tests::test_task_streaming_output	3	3.0s	3.0s
imix::bin/imix: tests::task_tests::test_task_streaming_output	3	3.0s	3.0s
imix::bin/imix: tests::task_tests::test_task_streaming_output	3	3.0s	3.0s

_{🎉 No failed tests in this run. | 🍂 No flaky tests in this run.}

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