Improve OpenOptions append+truncate error message

[BartSimpson001]

• I did not use an LLM to create a change in this PR.
• I used an LLM to create a change in this PR, and I have explained below how it was used.

An LLM (ChatGPT) was used to understand the codebase, locate the relevant platform-specific implementations and test files (unix.rs, windows.rs, and tests.rs), review the existing implementation, and assist in updating the test assertions to match the new error message. The final implementation, tests, and PR contents were manually reviewed and verified before submission.

Fixes #160716.

When OpenOptions is configured with both append(true) and truncate(true), the previous InvalidInput error message was:

creating or truncating a file requires write or append access

This message is misleading because append(true) already provides append access. This change replaces it with a more descriptive error message indicating that append and truncate cannot both be enabled at the same time.

The corresponding platform-specific tests have been updated to verify the new error message and ensure consistent behavior across the relevant implementations.

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• Owners of files modified in this PR: @ChrisDenton, libs
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