feat(mesh): add MeshSession singleton — ref-counted Zenoh session per process

[cagataycali]

Summary

Add strands_robots/mesh_session.py — a process-level singleton that manages a single zenoh.Session shared by all Mesh instances in the same process.

This is PR 1 of 6 for the Zenoh mesh feature (#95).

What this PR does

strands_robots/mesh_session.py (219 LOC)

A MeshSession class that provides:

Feature	Detail
Ref-counted	Multiple Mesh instances share one session; actual close at refcount=0
Fork-aware	Detects PID change and re-initialises (stale session after os.fork)
Lazy import	import zenoh only on first MeshSession.open() — zero cost at package import
Env config	ZENOH_CONNECT and ZENOH_LISTEN override endpoints
Programmatic config	config_overrides dict for CI/testing
atexit hook	Best-effort cleanup at interpreter shutdown
Fail-fast	RuntimeError if zenoh.open() fails — no silent fallback
Graceful degradation	Returns None if eclipse-zenoh not installed

Usage (future PRs will use this)

from strands_robots.mesh_session import MeshSession

session = MeshSession.open()    # refcount = 1
session2 = MeshSession.open()   # refcount = 2, same session
MeshSession.close()             # refcount = 1
MeshSession.close()             # refcount = 0, session.close() called

pyproject.toml changes

• Add [mesh] optional extra: eclipse-zenoh>=0.11.0,<1.0.0
• Include [mesh] in [all] extras
• Add zenoh.* to mypy ignore_missing_imports

Tests

15 tests in tests/test_mesh_session.py — all mock zenoh, no network required:

Category	Tests	What's verified
Lifecycle	5	open/close/refcount/reuse/noop
Import failure	1	Returns None when zenoh absent
Env config	3	ZENOH_CONNECT, ZENOH_LISTEN, multi-endpoint
Config overrides	1	Programmatic JSON5 injection
Fork detection	1	PID change → re-init
Open failure	1	RuntimeError propagation
Thread safety	1	10 concurrent open + 10 concurrent close
atexit	2	Cleanup + noop

Results: 15 passed, 0.4s. Full suite: 243 passed, 6 skipped, 0 failures.

Checklist

• Tests pass (pytest tests/test_mesh_session.py — 15/15)
• No regression (pytest tests/ — 243/243)
• Lint clean (ruff check + ruff format)
• Mypy clean
• Lazy import — no import zenoh at package level
• Fail-fast on unrecoverable errors
• Upper-bounded dependency (<1.0.0)
• 3 files changed, 522 insertions

Closes #95 (partially — PR 1 of 6)

---

🤖 AI agent response. Strands Agents. Feedback welcome!

[cagataycali]

Closing as duplicate of #101 — three concurrent mesh-session PRs were opened simultaneously (#100, #101, #103). Consolidating on #101 which has the most complete scope (4 files, 801 LOC incl. PeerInfo registry). If reviewers prefer pieces from this PR, we will cherry-pick before merging #101.

Canonical PR: #101
Tracking issue: #98