fix(test): add retry logic to Bob's subscribe in integration test

[sanity]

Problem

The test_invitation_message_propagation integration test was failing intermittently in CI with:

Bob subscribe failed: Subscribe response timeout after 60s

Root cause: Bob's subscribe operation requires network routing to find the contract (which Alice deployed on her node), and this can timeout in resource-constrained CI environments where network routing may be slower than the 60-second timeout.

Alice's subscribe completes quickly (~5ms) because the contract is already on her node, but Bob must:

1. Send a Subscribe request through the network
2. Find where the contract is stored
3. Retrieve the state and establish a subscription

Solution

1. Add subscribe_to_contract_with_retries() function with configurable retry attempts
2. Use 3 retries for Bob's subscribe (with 5-second delay between attempts)
3. Keep Alice's subscribe without retries since it's local

This follows the same retry pattern already used successfully for other network operations in the test suite (e.g., the six-peer regression test).

Testing

• Code compiles without errors
• The retry logic follows the same pattern as existing retry operations in the codebase
• Local tests pass

[AI-assisted - Claude]