test: fix pyright strict-mode errors in tests/test_observable/ (batch 1)

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Summary

This PR fixes pyright strict-mode type annotation errors across 16 test files in tests/test_observable/, targeting files with 5 or fewer errors each. This is batch 1 of the cleanup effort to eventually remove tests/test_observable/ from the [tool.pyright] exclude list in pyproject.toml.

Errors fixed: ~40 across 16 files

Patterns Fixed

Pattern	Files affected
Add (ex: Exception) -> None annotation to _raise helper	test_combinelatest, test_empty, test_interval, test_repeat, test_throw, test_timer
Delete unused _raise helper function	test_blocking, test_sample, test_throttlefirst
Add (ex: Exception) -> None to on_error callback	test_flatmap_async
Add type annotations to predicate, comparer, __init__	test_count, test_contains, test_pluck
Fix float vs int for TestScheduler ticks	test_timer (250.0 → 250)
Remove unused import	test_utility_fluent (import pytest)
Replace unused variable with _	test_windowing_fluent (evens, odds → _, odds)
Add # type: ignore[assignment] for single_or_default_async	test_filtering_fluent (6 lines) — the default_value: _T = None parameter widens _T to include None, causing Observable[int | None] to be inferred where Observable[int] is declared

Notes

• No production code changes — tests only
• pyproject.toml is not modified in this PR; that change (removing the exclude entry) will follow once enough test files are clean
• Related open PRs in this cleanup series: test: fix all ruff lint errors in tests/test_observable/; remove from ruff exclude #778 (ruff cleanup for test_observable), [Repo Assist] test: fix pyright strict-mode errors in tests/test_scheduler/; remove from pyright exclude #779 (pyright cleanup for test_scheduler)
• The # type: ignore[assignment] comments in test_filtering_fluent are a pragmatic fix. The root cause is that single_or_default_async(default_value: _T = None) infers _T = int | None when no argument is given. A proper fix would require changing the operator's overloads, which is a separate concern.

Testing

CI will validate with uv run pytest, uv run pyright, and uv run ruff check.

Note

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