mcp: every wait on the MCP path gets a bound, and a failed reload keeps what it had ...

reload_tools kept dying on prod, and only a restart cured it. Nothing on the
MCP path was bounded: ClientSession was built without read_timeout_seconds, so
the SDK waited forever (anyio.fail_after(None)); _send awaited a future with no
timeout; disconnect() awaited the shielded runner with no timeout; and
disconnect_all() walks the pool one client at a time, so a single wedged client
blocked every reload that came after it. A cancelled caller only cancelled
itself — the runner stayed wedged, which is why it was every reload, not one.

Bounds, in two deliberate layers. The SDK read timeout ends a request nobody
answers *inside* the runner, so the client stays usable; the caller-side bound
is the backstop for an op queued behind a stuck one, and it aborts the runner
rather than leaving it for the next command to queue behind. connect 60s,
list_tools 45s, call_tool 300s (SDK 240s), session read 30s, disconnect 15s,
each overridable, each logging the culprit.

disconnect() and disconnect_all() no longer swallow CancelledError — that is
exactly how the reload budget stopped being enforceable.

The registry is now rebuilt in one step: collect every server's tools first,
then commit the whole set in a single synchronous swap. clear_external_tools()
ran *before* the rebuild, so a rebuild that failed halfway left the model
holding MCP tool names that resolved to nothing. A failed pass now leaves the
previous registrations untouched, and the reload phase carries its own budget
so the tool always returns a report.

Two more defects fell out of the audit. The streamable_http transport was
handed a bare httpx.AsyncClient, whose 5s default read killed the GET stream of
an idle server — six of the nine prod servers are streamable_http. And
register_mcp_tools called log.warning() in a module where log was never
defined, a NameError on precisely the failure path it was meant to report.

reload: pick up the new code, and drop MCP tools that are gone ...

reload_tools reported success while three separate things kept it from
doing what it says.

The bytecode cache is keyed on (mtime in whole seconds, file size), so a
tool edited to the same length inside the same second as its previous
load re-ran the OLD code — the reload was real, the new version was not
live. The loader now compiles the source itself instead of consulting
__pycache__ (importlib.invalidate_caches() does not help here).

MCP registrations only ever grew: register_mcp_tools called
register_external, and unregister_external was used in one place, so a
server removed from the config or a tool a server stopped exposing
stayed in the registry and failed only when the model called it. Reload
now clears external tools and rebuilds them.

enabled.json naming a tool that failed to load (the gmail/html2text case)
was visible only as a log line. It is now part of the report.

The reload itself moves to navi/core/reload.py, one implementation shared
by the tool and the admin route, so the two cannot leave different
toolsets behind. list_tools.py also read enabled.json through its own
cwd-relative path, which made it disagree with the real toolset, and the
class-based loader rejected execute(self, params, ctx=None) — the shape
every built-in uses.