fix: nightly hardening - output format validation

[mouse-value-add]

Problem

PromptLens currently accepts any values in and silently skips unknown formats during export. This can hide configuration mistakes and produce incomplete artifacts without clear feedback.

Approach

• Added strict validation for against the supported set (, , , ).
• Normalized configured formats by trimming whitespace and lowercasing values to make config input more resilient.
• Added tests to verify normalization and explicit rejection of unsupported formats.

Verification

• Ran a direct Python validation check to confirm:
  • normalizes to
  • unsupported values like raise validation errors.
• Added to codify these checks.

Risks

• Configurations that previously used unsupported format names (and were silently ignored) will now fail fast.

Rollback Plan

• Revert this PR to restore previous permissive behavior, or relax validation to warnings if strict failure is too disruptive.