Preserve temporal serialization in reflected addition

[xiehuanyi]

Summary

timedelta + Date returns the correct date with an empty serialized value, so assigning it back to a document produces invalid TOML. timedelta + DateTime formats correctly but loses its original trivia. Both operations bypass the wrappers' existing addition methods.

Route reflected addition through the existing wrapped addition for both temporal types. Six cases cover forward/backward shifts of dates and naive/aware datetimes, matching ordinary addition's value, text and trivia and round-tripping the resulting document.

Validation: all six regressions fail on the unchanged source; all 1190 tests including conformance fixtures and applicable pre-commit checks pass after the change. The six existing Mypy diagnostics remain unchanged from the exact base, allowing for shifted line numbers.

Agent Drafting Metadata

• Agent: OpenAI Codex
• Model: GPT-6.1 Sol, xhigh
• Notes: This description and changes in tomlkit/items.py, tests/test_items.py and CHANGELOG.md were prepared with this agent.

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[Wkhanex]

Nine reflected-addition checks passed for dates, offset datetimes and local datetimes, including negative one-day, positive one-day and 27-hour-plus-one-microsecond deltas. I used Python 3.12.14 against commit 0b667c6.

For every case, delta + item and item + delta had the same TOML class, value and as_string output. Assigning the reflected result back into a commented document produced valid TOML with the comment intact, and stdlib tomllib decoded the expected temporal value. This checks the document-assignment path as well as the numeric result. I ran standalone assertions against fetched PR source; I did not run the full pytest suite.