fix: make StartConditions sync, drop block_in_place under ctrl_lock

[YuanYuYuan]

Summary

Fixes #2581. Under peer churn, the RX thread can deadlock the whole session.

The RX thread holds ctrl_lock. It calls block_in_place to wait on StartConditions, which is a tokio::sync::Mutex. At the same time, the Net runtime's single worker thread tries to acquire that same ctrl_lock, from Router::new_transport_unicast during gossip autoconnect. The worker cannot run. So the task that would release StartConditions never runs either. The session freezes.

This PR is a companion to #2637. #2637 fixes the same root cause with a different mechanism. See "Relationship to #2637" below.

The deadlock

• RX thread: holds ctrl_lock. Calls block_in_place on StartConditions. Two call sites: gossip.rs's link_states tail, and the peer hat's route_declare_final.
• Net worker (the runtime's only worker thread): blocked separately, trying to acquire ctrl_lock from Router::new_transport_unicast during gossip autoconnect.
• Nothing releases StartConditions, because the one worker that could run is stuck on ctrl_lock. The session wedges: puts freeze, transport leases die.

The fix

Every StartConditions critical section is a plain Vec push or drain. None of them await anything. So peer_connectors becomes a std::sync::Mutex instead of a tokio::sync::Mutex. Every method on StartConditions becomes synchronous. Both block_in_place call sites are deleted, not rescheduled.

Relationship to #2637

#2637 takes a different approach at the same two sites: spawn the notification as a task on ZRuntime::Net, instead of blocking on it. That removes the RX thread from this specific cycle. But the spawned task still needs ZRuntime::Net's one worker to run. Nothing in that fix adds a second worker, and nothing removes the single-worker precondition that caused the deadlock. If that worker is ever blocked elsewhere, the spawned task queues indefinitely. If a future caller blocks synchronously on the task's result, the same deadlock returns, with one extra hop.

This PR removes the scheduling dependency instead. There is no .await left inside StartConditions for anything to be rescheduled onto.

@otamachan's independent reproduction on #2637 goes through rmw_zenoh_cpp and ROS 2 directly, with a full five-thread backtrace. It confirms both #2637's approach and this PR's approach target the same two block_in_place sites. Worth reading alongside this PR.

Credit

This PR ports a fix independently developed and field-validated by Guillaume Doisy (Dexory): botsandus/zenoh@ff6cc2bc3. Authorship is preserved on the commit (Author: Guillaume Doisy <guillaume@dexory.com>). Their original commit message, quoted for the record:

Seen in production as random ROS 2 process freezes on 16-25% of bringups.

This PR ports that fix onto current main. It adapts it past the #2096 hat rename (p2p_peer to peer) and the resulting route_declare_final signature change. No other logic changed.

Testing

• cargo fmt --check -p zenoh: clean
• cargo clippy -p zenoh --lib -- -D warnings: clean. Five pre-existing warnings remain in unrelated files, untouched by this diff.
• cargo test -p zenoh --lib net::runtime::orchestrator: 3 of 3 pass
• No new test covers the deadlock itself. The field evidence cited above already covers it at a scale (about 1,700 restart cycles, plus an independent 200Hz A/B test) that a unit test here cannot easily reproduce. Happy to add a regression test if maintainers want one in a specific shape.

[codecov]

Codecov Report

❌ Patch coverage is 94.44444% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 74.76%. Comparing base (b828c6c) to head (49c2dd9).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
zenoh/src/net/protocol/gossip.rs	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2779      +/-   ##
==========================================
+ Coverage   74.72%   74.76%   +0.04%     
==========================================
  Files         419      419              
  Lines       63955    63934      -21     
==========================================
+ Hits        47790    47803      +13     
+ Misses      16165    16131      -34     

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[doisyg]

ECA validated

[YuanYuYuan]

ECA validated

@doisyg Thanks!

@JEnoch Could you please review this?

[JEnoch]

LGTM but I would like also @OlivierHecart to review, as he's the architect of this code part.