Document tt and HH/H incompatibility in parsing format strings

[Copilot]

• Add a note to the "tt" custom format specifier section explaining that when parsing, you must use "hh"/"h" (12-hour clock) instead of "HH"/"H" (24-hour clock), because they're incompatible and will throw a FormatException
• Add a note to the "HH" custom format specifier section mentioning that it's incompatible with the "tt" AM/PM designator in parsing operations (throws FormatException)
• Use "throws" instead of "causes" for FormatException in both notes
• Add ai-usage: ai-assisted to frontmatter

Original prompt

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This section details on the original issue you should resolve

<issue_title>When parsing with "tt", it requires using "hh"</issue_title>
<issue_description>According to stackoverflow (and this matches my tests locally), you can't call DateTime.ParseExact with both HH and tt. Instead you must use hh.

https://stackoverflow.com/questions/2596969/why-can-t-datetime-parseexact-parse-the-am-pm-in-4-4-2010-42000-pm-using

This would be good to mention in the docs.

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Document Details

⚠ Do not edit this section. It is required for docs.microsoft.com ➟ GitHub issue linking.

• ID: dda93f14-f1b5-fde6-e441-302cdac261ab
• Version Independent ID: 96abc7c4-3719-82da-e271-881d62f0640f
• Content: Custom date and time format strings
• Content Source: docs/standard/base-types/custom-date-and-time-format-strings.md
• Product: dotnet-fundamentals
• GitHub Login: @adegeo
• Microsoft Alias: adegeo</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes When parsing with "tt", it requires using "hh" #28140

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[Copilot]

Pull request overview

Adds documentation clarifying that 24-hour hour specifiers (H/HH) are incompatible with AM/PM designators (t/tt) when parsing with exact-format APIs (for example, DateTime.ParseExact), and that mixing them results in FormatException.

Changes:

• Adds a [!NOTE] under the "HH" specifier warning against combining H/HH with t/tt in parsing format strings.
• Adds a symmetric [!NOTE] under the "tt" specifier recommending h/hh for parsing when an AM/PM designator is present.