fix(pricingRegistry): normalize model names to lowercase for case-insensitive lookup

[Srejoye]

Problem

register() stored model names exactly as passed, while getPricing() and hasPricing() looked them up exactly as passed. JavaScript Map keys are case-sensitive, so registerPricing("openai", "GPT-4o", ...) followed by a lookup for "gpt-4o" (the format API responses use) silently failed.

The error from getPricing() was caught upstream in the cost engine, which returned $0 — causing the API call to go completely untracked with no warning to the user.

Root Cause

src/core/pricingRegistry.ts — three methods:

• register() line 120: set(model, pricing) — no normalization
• getPricing() line 134: providerPricing.get(model) — no normalization
• hasPricing() line 147: .has(model) — no normalization

Provider names were normalized (provider.toLowerCase()) but model names were not, creating an inconsistent API surface.

Fix

Add const normalizedModel = model.toLowerCase() in all three methods and use it for Map operations. Error messages retain the original model string for readable diagnostics.

Compatibility

All built-in model registrations in initializeDefaultPricing() already use lowercase strings — existing behavior is 100% preserved. This only affects users who called registerPricing() with mixed-case model names.

Behaviour

Scenario	Before	After
register("openai", "GPT-4o") then getPricing("openai", "gpt-4o")	throws → silent $0	returns pricing ✓
hasPricing("openai", "GPT-4O")	false	true ✓
All built-in model lookups	work	work ✓ (unchanged)
Provider name case-insensitivity	works	works ✓ (unchanged)

Closes #7