warning check and test coverage

[nautilus69]

Convert check_simulation errors to warnings and add validate_simulation (#68, #93)

Issue #68: Replace hard errors with warnings in check_simulation to allow
incremental building of simulations. Users can now add simulations without
all building blocks (protocols, formulations) being present upfront.
A new validate_simulation() method provides strict pre-execution validation
that throws errors when required components are missing.

Issue #93: Add comprehensive tests for output selections and observer paths,
including default selections, set/add operations, and snapshot round-trip
preservation.

Changes:

• R/simulations.R: cli_abort -> cli_warn for missing formulation checks
• R/snapshot.R: check_simulation() now warns instead of erroring;
  new validate_simulation() method for strict validation before runs
• tests/testthat/test-simulation.R: convert expect_error to expect_warning
  for check_simulation tests; add 10+ test cases for validate_simulation;
  add 5+ test cases for output selections/observers
• Updated roxygen docs and test snapshots

This will resolve both #93 and #68

[dlefaudeux]

Would be nice if validate_simulation also says the name of the simulation for which there is an issue. Because it will be called before the run_simulations an otherwise it just says that is missing but not for which sim.

[dlefaudeux]

And for #93 I meant that it is supported to use as outputs in the output selection thing that are defined in the Observer building block