Fix crash in fixer on certain invalid patterns

[danmoseley]

Fix #80004

1. Add failing test and make fix.
2. Tweak two strings.

Edit: actually several. I see no reason why double backslashes are in the strings, they appear literally in the exception messages. Fixed them all.

[ghost]

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Issue Details

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Fix #80004

1. Add failing test and make fix.
2. Tweak two strings.

Author:	danmoseley
Assignees:	-
Labels:	area-System.Text.RegularExpressions
Milestone:	-

[danmoseley]

Failure is known and not relevant.

[danmoseley]

of the failures both WASM

https://helixre107v0xdeko0k025g8.blob.core.windows.net/dotnet-runtime-refs-pull-80007-merge-1b7fbe7a837c425684/normal-System.Runtime.Tests/1/console.3b695942.log?helixlogtype=result is another instance of #79874

the other is a timeout https://helixre107v0xdeko0k025g8.blob.core.windows.net/dotnet-runtime-refs-pull-80007-merge-351a8f2b805347769a/WasmTestOnBrowser-System.Runtime.Tests/1/console.708fc41e.log?helixlogtype=result

[05:33:21] info: Initializing dotnet version 8.0.0-ci commit hash 5bb27278b4849e929ed6e146d79568abffcc408a
[05:33:21] info: Discovering: System.Runtime.Tests.dll (method display = ClassAndMethod, method display options = None)
[05:33:24] info: Discovered:  System.Runtime.Tests.dll (found 8930 of 9077 test cases)
[05:33:24] info: Using random seed for test cases: 575081098
[05:33:24] info: Using random seed for collections: 575081098
[05:33:24] info: Starting:    System.Runtime.Tests.dll

...
[EXECUTION TIMED OUT]
Exit Code:-3Executor timed out after 3600 seconds and was killed

this is not relevant to this change as none of this code uses the regex generator or runs the regex tests.

[buyaa-n]

LGTM, thank you!