feat(action-plugin): run dynamic-run over RPC - #16256

Replace the temporary-file dynamic-run handshake with versioned DAP RPC
procedures. Dune registers each active action, passes its RPC endpoint and
action ID to the plugin, and serves dependency requests while staged execution
continues in one process.

Provide a monad-parametric DAP client with explicit structured errors, plus a
blocking implementation backed by the shared thread pool. Keep the DAP payload
representations private while exposing the request witnesses required by
caller-provided Dune RPC clients.

Record RPC-built dependencies in the action result and remove the obsolete
action-execution continuation used to respawn plugins between stages. Update
tests, procedure digests, and documentation for single-process staging.

Signed-off-by: Rudi Grinberg <me@rgrinberg.com>

Extend the action-plugin helper so a rule producing one target requests a
second target whose rule depends on the first. Record the existing timeout to
demonstrate that serving the dependency from an unrelated RPC fiber loses the
Memo cycle-detection path and deadlocks.

Signed-off-by: Rudi Grinberg <me@rgrinberg.com>

Capture the Memo cycle-detection context before action execution and restore it
when an RPC handler builds dynamic dependencies. This reconnects the handler's
build to the active rule's Memo stack, allowing indirect cycles to be detected
before either side blocks.

Use the general Memo cycle detector for direct target dependencies as well,
removing the duplicate target matching logic and restoring Dune's standard
cycle diagnostics.

Signed-off-by: Rudi Grinberg <me@rgrinberg.com>

Extend the blocking action-plugin helper to hold an initialized action open and
publish its action ID and RPC endpoint. Start a second blocking client with the
same ID to demonstrate that the server currently accepts a live action from an
unrelated RPC session.

Signed-off-by: Rudi Grinberg <me@rgrinberg.com>

Generate unpredictable action IDs instead of process-local counters and bind an
action to the RPC session that initializes it. Reject duplicate initialization,
uninitialized dependency requests, and requests from any other session.

Update the blocking-client regression to verify that a second session can no
longer reuse a live action ID.

Signed-off-by: Rudi Grinberg <me@rgrinberg.com>