fix(v2): don't mistake a literal '[' inside a preceding string field for the tasks array start

[roli-lpci]

Describe your changes

IterableBase.tasks_from_chunks / tasks_from_chunks_async (instructor/v2/dsl/iterable.py), the
streaming path every provider's Iterable[Model] extraction goes through, detects the start of the
streamed tasks array with a naive "[" in chunk check on each individual chunk. If a field
preceding tasks in the response JSON contains a literal [ inside a string value — for example a
free-text note field like "check [priority] items first" — that in-string bracket falsely
triggers array-start detection. The buffer gets sliced from the wrong offset, and every task
streamed after that point is silently dropped: the function returns an empty list, with no
exception anywhere.

chunks = [
    '{"note": "check [priority] items first", ',
    '"tasks": [',
    '{"name": "alpha", "priority": 1}',
    ', {"name": "beta", "priority": 2}',
    "]}",
]
list(IterableModel(Task).tasks_from_chunks(chunks))
# []  <- both tasks vanished, no error

The fix adds _find_unquoted_char, a small string-aware scanner (tracks in_string/escape state,
the same pattern get_object a few lines below already uses for brace-matching) and uses it in
both tasks_from_chunks and tasks_from_chunks_async to find the real, unquoted [ against the
full accumulated buffer instead of a naive per-chunk substring check. 38 lines changed across both
sync/async entry points plus the new helper; no signature changes.

Tests: added test_iterable_tasks_from_chunks_ignores_bracket_inside_preceding_string and its
async counterpart, mirroring the existing test_iterable_tasks_from_chunks_handles_braces_inside_strings
pattern, using the note-field repro above. Before fix: 2 failed — assert [] == [Task(name='alpha', priority=1), Task(name='beta', priority=2)] (both streamed tasks silently dropped). After fix: all
6 tests in tests/v2/test_iterable_streaming.py pass. ruff check is clean on both changed files.

Issue ticket number and link

No existing issue tracks this bug. Distinct from the already-merged PR #2340 (a consolidation
bundle whose iterable.py portion fixed brace-in-string detection in get_object) — this fixes a
different detection bug (array-start bracket, not object-boundary brace) in a different function;
get_object itself is unchanged.

Checklist before requesting a review

• I have performed a self-review of my code
• If it is a core feature, I have added thorough tests.
• If it is a core feature, I have added documentation. (Bug fix to internal streaming
  detection; no documented behavior changed.)