fix: nightly hardening - provider normalization

[mouse-value-add]

Problem

Provider lookup currently lowercases but does not trim whitespace. Inputs like fail with a not-supported error, and blank provider strings fail with less-actionable messaging.

Approach

• Normalize provider names with before registry lookup.
• Add explicit blank-provider guard with a clear error.
• Return a stable, sorted provider list in error output for easier debugging.
• Add regression tests covering whitespace/case normalization and blank provider rejection.

Verification

• Listing 'promptlens'...
  Listing 'promptlens/exporters'...
  Listing 'promptlens/judges'...
  Listing 'promptlens/loaders'...
  Listing 'promptlens/models'...
  Listing 'promptlens/providers'...
  Listing 'promptlens/runners'...
  Listing 'promptlens/utils'...
  Listing 'tests'...
• Manual runtime assertions for:
  • resolves to
  • raises with actionable message

Risks

Low risk. Behavior only changes for malformed provider strings and improves compatibility for whitespace-padded values.

Rollback Plan

Revert this PR to restore previous provider lookup behavior in and remove the added hardening tests.

[brainsparker]

Closing: this change has landed on main via the consolidation in #40, which merged the best version of each open hardening PR in one reviewed change set. Thanks - the work itself is merged.