Fixes & improvements to Expand behaviour

[Porges]

• Recursively expand Paths before canonicalization; fixes Expand fails to work recursively with some combinations #2387
• Detect and error on infinitely-expanding variables (due to self-reference or mutual recursion); this fixes an existing problem with list expansion as well
• Detect and report references to unknown replacements (e.g. typoes)
• Detect and report references to known replacements which are not available

---

There is a behavioural change here which is that values which were previously unknown such as {x} were unexpanded; from now on these will be errors. It's possible that something somewhere is relying on this.

[codecov-commenter]

Codecov Report

Merging #2789 (081aeb3) into main (f402304) will increase coverage by 0.37%.
The diff coverage is 94.88%.

@@            Coverage Diff             @@
##             main    #2789      +/-   ##
==========================================
+ Coverage   29.14%   29.52%   +0.37%     
==========================================
  Files         322      322              
  Lines       38607    38782     +175     
==========================================
+ Hits        11252    11449     +197     
+ Misses      27355    27333      -22     

Impacted Files	Coverage Δ
...rc/agent/onefuzz-task/src/tasks/coverage/dotnet.rs	0.00% <0.00%> (ø)
...c/agent/onefuzz-task/src/tasks/coverage/generic.rs	0.00% <0.00%> (ø)
src/agent/onefuzz/src/expand.rs	82.28% <96.40%> (+13.80%)	⬆️

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