handle ranged parallel awaiter with empty awaiters

[wokron]

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[Copilot]

Pull request overview

This PR updates ranged parallel awaiters to correctly handle empty awaiter ranges, preventing hangs and defining behavior for when_all/when_any on empty inputs.

Changes:

• Make ranged parallel awaiters complete immediately (await_ready() == true) when the input range is empty.
• Make ranged when_any extraction use bounds-checked access to produce a defined exception on empty input.
• Add tests covering when_all and when_any with empty ranges.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
tests/test_awaiter_operations.cpp	Adds test cases for when_all/when_any with empty ranges (immediate completion vs. exception).
include/condy/finish_handles.hpp	Uses order.at(0) to define std::out_of_range behavior for empty ranged when_any.
include/condy/awaiters.hpp	Treats empty ranged awaiter collections as immediately-ready to avoid deadlock/hang.

Comments suppressed due to low confidence (1)

include/condy/finish_handles.hpp:316

• The comment suggests it's the caller's responsibility to ensure the range is non-empty, but the current behavior (and the new test) relies on extract_result() throwing std::out_of_range for an empty range. Please update this comment to match the intended API contract (e.g., explicitly document that empty input throws), or handle the empty case explicitly in a way that aligns with the public when_any(range) semantics.

        // May throw out_of_range if the range is empty, but it's the caller's
        // responsibility to ensure that.
        auto idx = order.at(0);
        return std::make_pair(idx, std::move(results[idx]));

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