Improve ServerBuilding teaching logic, add safety checks and introduce two new validation tests

[JaskRendix]

Changes:

• added a bidirectional consistency check between teacher and student to prevent stale or mismatched references, when a mismatch is detected, both sides of the relationship are cleaned up safely
• improved student assignment logic so that a teacher only clears its work type when necessary, avoiding redundant colony production recalculations
• clarified and documented the teaching flow, including how training turns are incremented and when they are reset
• strengthened csTrainStudent by ensuring both teacher and student have their moves and training counters reset in a consistent and predictable way
• improved logging and comments to make the teaching lifecycle easier to understand and debug
• modernized the unit‑repair logic using a clearer stream‑based implementation
• simplified csCheckMissingInput with early returns and clearer branching between “no input at all” and “insufficient input” cases
• ensured csNewTurn logs the building state before performing teaching and repair operations
• added two new tests to validate teaching behavior: a test ensuring that a student who already has the same skill as the teacher is not assigned for training + a test ensuring that no student is assigned when the colony contains no eligible units

Things I considered:

• I checked how teaching interacts with food, production, and work assignments. None of these affect training; as long as there’s a valid student, teaching just continues.
• I looked into whether buildings can be disabled or damaged in a way that would stop teaching or repairs. The ruleset doesn’t support that idea, so buildings always function if they exist.
• I reviewed whether modifiers or colony bonuses could change training speed or eligibility. Training times are fixed by unit type and school level, with no dynamic adjustments.
• I examined how student selection works when several candidates are available. The engine handles this internally, and there’s no way to force a specific choice in tests.
• I confirmed that units working in production buildings can still be reassigned as students. The system is designed to pull them out immediately if needed.
• I focused on the core teaching rules and added two tests that match them: one to ensure a unit already at the teacher’s skill level isn’t selected, and one to ensure no student is assigned when none are eligible