fix(tools): keep the forget-memory timeout when a caller passes a signal

[addyCooks]

Closes #1549

Problem

#1451 bounded DELETE /v4/memories with a 30-second abort, but the caller
signal and the timeout were selected between with ??:

signal: options?.signal ?? AbortSignal.timeout(FETCH_TIMEOUT_MS)

They were mutually exclusive. Passing a cancellation signal silently dropped
the timeout and the request became unbounded again - the exact hang #1451 set
out to remove - with no way to ask for both through the API. Latent today
(ai-sdk.ts:332 and openai/tools.ts:490 both omit options), live the
moment anyone wires up cancellation.

Fix

Compose the two with AbortSignal.any, building the timeout signal once so
the bare path reuses it. AbortSignal.any is available in Node 20+ (the
repo's engines floor), Bun, and workerd.

Test

tool-operations.test.ts replaces the case that asserted the old select-one
behaviour with five:

• the caller signal is composed rather than substituted, and its abort reason
  reaches the request
• the timeout still fires while the caller signal stays open
• an already-aborted caller signal is forwarded with its reason
• the bare path still receives the 30s timeout signal itself, not a composition
• an aborted fetch surfaces to the caller rather than being swallowed

18 tests in the file pass. The timeout case spies on AbortSignal.timeout
(shortened to 5ms) while asserting the code still requested 30_000 - Node
captures its internal timer at module load, so fake timers cannot intercept
AbortSignal.timeout.

Each case was checked against mutants of the fix, so the suite fails for a
wrong-but-plausible fix and not only for a full revert:

mutant	result
AbortSignal.any([options.signal]) - timeout leg dropped	1 case fails
composed with a signal that never fires	1 case fails
revert to options?.signal ?? ...	2 cases fail
timeout shortened to 300ms	2 cases fail
the fix as submitted	18/18 pass

Verification against a real socket

Against a server that accepts the connection and never responds:

scenario	before	after
caller signal present, never aborts	hangs indefinitely	TimeoutError at the deadline
no caller signal	TimeoutError	TimeoutError
caller aborts early	AbortError	AbortError
already-aborted caller signal	immediate AbortError	immediate AbortError
200 response with a caller signal	resolves	resolves

Note (not in this PR)

apps/mcp/src/server/client/index.ts:370 (getDocuments) has the identical
?? pattern and is equally latent - no caller there passes options either.
Left out to keep this diff scoped to the issue and to avoid colliding with
#1564, which already touches that file.