fix(core): return a matched RunnableBranch's falsy output instead of the default

[Khizar Chaudhry (Khizarc)]

Fixes #11583

Problem

RunnableBranch._invoke decided whether a branch had run by testing its output:

if (conditionValue) {
  result = await branchRunnable.invoke(...);
  break;
}
if (!result) {                       // <-- 0, "", false, null all land here
  result = await this.default.invoke(...);
}

A branch that matched and returned a falsy value is indistinguishable from no branch
matching at all, so its output was discarded and the default ran instead. No error is
raised, so the call silently returns the wrong value:

const branch = RunnableBranch.from([
  [(x: number) => x > 0, (x: number) => 0],
  (x: number) => -1,
]);
await branch.invoke(5);   // expected 0, got -1

Fix

Track whether a branch matched instead of inferring it from the output.

The streaming path at _streamIterator already avoids this with stream === undefined,
which is safe there because a stream object is never falsy. I used an explicit flag
here rather than mirroring that check, because a branch may legitimately return
undefined — result === undefined would still fall through in that case. There is a
test covering it.

Tests

Added to runnable_branch.test.ts:

• a matched branch returning 0, "", false or null is returned, not the default
• a matched branch returning undefined is returned, not the default
• the default is still used when no condition matches
• batch preserves falsy branch outputs

Verified these fail before the change and pass after:

	before	after
runnable_branch.test.ts	3 failed, 5 passed	8 passed

Full src/runnables/tests/ suite: 170 passed, with the same 2 pre-existing
streamEvents failures present on a clean checkout. No type errors.

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