fix Linux daemon PID lookup without lsof

[koriyoshi2041]

Complements #3585 for minimal Linux images where neither lsof nor ss is installed.

PID discovery now falls back to procfs: it finds listening TCP/TCP6 socket inodes for the target port in /proc/net, then resolves the owning process through /proc/<pid>/fd symlinks. Existing lsof and ss paths remain preferred.

Validation after rebasing onto current main:

• uv run pytest tests/test_embed_manager.py -q — 28 passed
• env -u HTTP_PROXY -u HTTPS_PROXY -u ALL_PROXY -u http_proxy -u https_proxy -u all_proxy NO_PROXY=127.0.0.1,localhost uv run pytest tests/ -q — 187 passed
• uv run ruff check hindsight_embed tests — passed
• uv run ruff format --check hindsight_embed tests — 22 files already formatted
• git diff --check — passed

The proxy variables are removed for the full suite because the local environment points HTTPX at a SOCKS proxy without its optional socksio dependency; the first run otherwise produced 18 control-center transport setup failures unrelated to this patch.

[ebarkhordar]

Ran this against a real listener rather than the mocked subprocess.run, since CI's test-embed job is skipped on this PR (it is gated on has_secrets, which is false for a fork), so the new test has not executed upstream.

python:3.12-slim plus iproute2, lsof absent, a real socket bound to 127.0.0.1:9177, the package pip-installed from each side:

• base 205e47b4: _find_pid_on_port(9177) returns None
• head b69873f3: returns 342, which is the listener's own pid
• tests/test_embed_manager.py at head: 22 passed

So the ss -H -ltnp "sport = :9177" argv is valid as written (iproute2 6.15.0), which the mocked test cannot show on its own.

One gap, not blocking. #3517 lists minimal containers next to CachyOS, and those often have neither binary. On stock python:3.12-slim (no lsof, no ss) head still returns None, and since #3171 a missing pid makes stop() return False rather than only log a warning. A stdlib fallback would cover that case with no new dependency: read /proc/net/tcp and /proc/net/tcp6 for the listening socket's inode (state 0A), then match it against the /proc/[0-9]*/fd/* symlinks. I tried that in a container with neither binary and it resolved the correct pid. Fine as a follow-up if you would rather keep this PR to the environment that was reported.

[koriyoshi2041]

Added a stdlib /proc fallback in 80d75f5 for Linux systems with neither lsof nor ss. It matches LISTEN entries in /proc/net/tcp{,6} to /proc/<pid>/fd socket inodes; focused embed-manager coverage is now 24/24, with source lint, format, and diff checks clean.