fix: inconsistent assertion error message formatting across 86+ files

[thomhurst]

Description

Error messages across the assertion library use inconsistent prefixes, creating a fragmented user experience:

• StringAssertions.cs:101 → "found \"{value}\""
• NullAssertion.cs:51 → "value is null"
• CollectionAssertions.cs:35 → "collection was null"
• CollectionAssertions.cs:107 → "value was null"
• GreaterThanAssertion.cs → "value is null" vs "found {value}"
• BetweenAssertion.cs:81 → "value is null" vs line 95 "found {value}"

Impact

Inconsistent error messages reduce clarity and create cognitive friction. Users expect similar assertions to provide similarly-formatted feedback.

Suggested Fix

Establish a standard error message format:

• "found X" for actual values
• "was null" for null values
• "threw ExceptionName" for exceptions

Consider creating a helper method like CreateErrorMessage(string actual) to centralize formatting.