SMT2 Incremental: Implement push/pop interface

[tautschnig]

Replace UNIMPLEMENTED_FEATURE stubs in push(), push(assumptions), and pop() with working implementations that send SMT2 push/pop commands to the solver process.

• push() sends smt_push_commandt{1} and increments context counter
• pop() sends smt_pop_commandt{1} and decrements context counter, with PRECONDITION guard against popping from empty stack
• push(assumptions) calls push() then set_to for each assumption
• Add unit tests covering all three methods
• Remove no-new-smt tag from fault_localization-stop_on_fail1 test

Fixes: #8073

• Each commit message has a non-empty body, explaining why the change was made.
• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.42%. Comparing base (0170409) to head (c7145ba).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8908      +/-   ##
===========================================
+ Coverage    80.41%   80.42%   +0.01%     
===========================================
  Files         1703     1703              
  Lines       188398   188416      +18     
  Branches        73       73              
===========================================
+ Hits        151496   151535      +39     
+ Misses       36902    36881      -21     

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