feat: multi-provider pricing and custom rate overrides

[rajkumarsakthivel]

Summary

• Added static pricing for 15 models across OpenAI, Google, and Anthropic so savings estimates work for non-Claude users
• Added pricing.input and pricing.output config overrides in cce.toml for unlisted or custom-priced models
• Dashboard savings page now shows estimated cost saved, active pricing model, and rates
• Anthropic pricing still auto-fetches from docs (live data wins over static fallbacks)

Example cce.toml configs:

# Use a known model
[pricing]
model = "gpt-4o"

# Override rates for any model
[pricing]
model = "deepseek-v3"
input = 0.27
output = 1.10

Closes #99

[rajkumarsakthivel]

Screenshot

New "Est. Cost Saved" card showing $11.18 saved at opus rates. Pricing note below shows active model and rates with config hint.

[Copilot]

Pull request overview

Adds multi-provider model pricing support so savings/cost estimates work beyond Anthropic defaults, with config-driven model selection and per-rate overrides, and surfaces this information in both CLI and dashboard.

Changes:

• Introduces static pricing for multiple OpenAI/Google/Anthropic models and merges it with live-fetched Anthropic pricing (cache-backed).
• Adds pricing.input / pricing.output overrides to config and uses a shared resolve_pricing() helper in CLI/dashboard.
• Extends the dashboard savings API/UI and updates tests accordingly.

Reviewed changes

Copilot reviewed 7 out of 8 changed files in this pull request and generated 7 comments.

Show a summary per file

File	Description
tests/test_cli_smoke.py	Updates pricing fallback expectations and asserts presence of non-Anthropic models.
tests/dashboard/test_server.py	Extends /api/savings assertions to include pricing metadata and cost saved.
src/context_engine/pricing.py	Adds static multi-provider pricing, merged lookup behavior, and resolve_pricing() / list_available_models().
src/context_engine/dashboard/server.py	Adds pricing/cost fields to /api/savings.
src/context_engine/dashboard/_page.py	Updates Savings page UI to show estimated cost saved and pricing note.
src/context_engine/config.py	Adds optional pricing rate override fields and config mapping/type validation.
src/context_engine/cli.py	Switches savings reporting to use centralized resolve_pricing().

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