Fix unreachable panic in parser

[dylwil3]

Parsing the (invalid) expression f"{\t"i}" caused a panic because the TStringMiddle character was "unreachable" due the way the parser recovered from the line continuation (it ate the t-string start).

The cause of the issue is as follows:

The parser begins parsing the f-string and expects to see a list of objects, essentially alternating between interpolated elements and ordinary strings. It is happy to see the first left brace, but then there is a lexical error caused by the line-continuation character. So instead of the parser seeing a list of elements with just one member, it sees a list that starts like this:

• Interpolated element with an invalid token, stored as a Name
• Something else built from tokens beginning with TStringStart and TStringMiddle

When it sees the TStringStart error recovery says "that's a list element I don't know what to do with, let's skip it". When it sees TStringMiddle it says "oh, that looks like the middle of some interpolated string so let's try to parse it as one of the literal elements of my FString". Unfortunately, the function being used to parse individual list elements thinks (arguably correctly) that it's not possible to have a TStringMiddle sitting in your FString, and hits unreachable.

Two potential ways (among many) to solve this issue are:

1. Allow a TStringMiddle as a valid "literal" part of an f-string during parsing (with the hope/understanding that this would only occur in an invalid context)
2. Skip the TStringMiddle as an "unexpected/invalid list item" in the same way that we skipped TStringStart.

I have opted for the second approach since it seems somehow more morally correct, even though it loses more information. To implement this, the recovery context needs to know whether we are in an f-string or t-string - hence the changes to that enum. As a bonus we get slightly more specific error messages in some cases.

Closes #18860

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

Formatter (stable)

ℹ️ ecosystem check encountered format errors. (no format changes; 1 project error)

openai/openai-cookbook (error)

warning: Detected debug build without --no-cache.
error: Failed to parse examples/mcp/databricks_mcp_cookbook.ipynb:12:1:8: Simple statements must be separated by newlines or semicolons

Formatter (preview)

ℹ️ ecosystem check encountered format errors. (no format changes; 1 project error)

openai/openai-cookbook (error)

ruff format --preview

warning: Detected debug build without --no-cache.
error: Failed to parse examples/mcp/databricks_mcp_cookbook.ipynb:12:1:8: Simple statements must be separated by newlines or semicolons

[dylwil3]

@MichaReiser okay after a more thorough investigation I have a better understanding of what is going on here and have gone with a different fix. I've tried to summarize my reasoning in the updated PR summary. Let me know whether you agree, or whether you'd like me to solve the issue in a different way.

[MichaReiser]

Makes sense. Thank you