Broadcast unexpected behavior fix

[Aryan-Railtown]

Closes #887

What does this add?

This PR improves the reliability of the pubsub system by changing subscriber execution from concurrent to sequential inside the publisher.

Previously, subscribers were executed using:

await asyncio.gather(*contracts)

During session shutdown, this could result in subscriber tasks being cancelled before completion, causing certain callbacks (e.g., broadcast callbacks used for observability) to silently fail.

This PR changes execution to:

for contract in contracts:
    await contract

This ensures subscribers complete deterministically and prevents callbacks from being interrupted during shutdown.

---

Type of changes

• 🐛 Bug fix (non-breaking change which fixes an issue)

---

Background context

While investigating issues with rt.broadcast(), we discovered that pubsub subscribers were executed concurrently via asyncio.gather. During session shutdown this could lead to subscriber tasks being cancelled before they finished executing, causing broadcast callbacks to never trigger.

Although rt.broadcast() is moving away from being exposed as a public API (with the newer Flow-based execution model handling observability), sequential subscriber execution improves overall reliability of the pubsub pipeline and prevents silent callback failures.

---

Checklist for Author

Code Quality

• Code follows the project's style guidelines (run ruff check . and ruff format .)
• Code is commented, particularly in hard-to-understand areas

Testing

• Tests added/updated and pass locally (pytest tests)
• Test coverage maintained

Documentation

• Documentation updated if needed (bot will verify)

Git & PR Management

• PR title clearly describes the change

[soulFood5632]

LGTM, thanks for the investigation