ExpressionAssistant: improve fence parsing

[labkey-nicka]

Rationale

This seeks to address #1487 by improving edge case detection and handling for fence parsing.

Related Pull Requests

• https://github.com/LabKey/premiumModules/pull/711

Changes

• Fence parsing: accept any backtick length, use only the first info-string token, and scan all responses at once.
• Prompt fix: the four-backtick example could lead the model to emit fences the parser missed.
• Log the raw response at debug, and warn when a validator payload renders as prose instead of an apply action.
• Unit coverage for the fence shapes and leak detector