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input(): forward prompt through Conductor and flush pending stdout (#254)

* Forward input()'s prompt through Conductor and flush pending stdout

Completes the py-slang side of source-academy/py-slang#190: input()
was already blocking correctly via conductor.requestInput(), but
discarded its prompt argument entirely and never flushed stdout
buffered by earlier print()s before blocking.

- MiscBuiltins.input now accepts 0-1 args (@Validate(0, 1, ...)),
  writes the prompt to stdout (matching CPython's input(prompt)
  behavior) and forwards it to receiveInput/conductor.requestInput.
- createInputStream gained a setNextPrompt side-channel so its
  pull()-driven ReadableStream can pass a per-call prompt through to
  requestInput(), since pull() itself has no visibility into
  per-call arguments.
- receiveInput now flushes any buffered stdout before blocking, so
  print()s issued earlier in the same chunk aren't stuck in the
  buffer while the popup waits.
- Context.streams gained an optional flushStdout hook, wired up in
  PyCseEvaluator.evaluateChunk.
- Added src/tests/input.test.ts covering both the with- and
  without-prompt paths.

Depends on the (as yet unpublished) conductor protocol change adding
requestInput(prompt?). Points @sourceacademy/conductor at the local
checkout via a portal: resolution for now; repin to a real published
version once that lands upstream. Note: the portal symlink currently
breaks this repo's jest.config.js (its moduleNameMapper/
transformIgnorePatterns assume a real node_modules directory, not a
symlink resolving outside rootDir) - verified tests by temporarily
copying the built conductor package into node_modules instead; no
action needed once a real release replaces the portal link.

* Fix eager pull() firing a phantom requestInput before any input()

ReadableStream defaults to highWaterMark: 1, which makes it eagerly
call pull() as soon as it's constructed - i.e. during evaluator
init, before any Python input() call has... (continued)

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