[observe] Add getCost() formatted output helper

[nagasatish007]

Summary

Add a formatCost(summary: ObserveCostSummary): string utility that pretty-prints cost data for terminal output.

What to do

• Create src/observe/format-cost.ts
• Format output: "$0.0023 (2 requests) | Input: $0.0018 | Output: $0.0005"
• Export from tealtiger/observe
• Add unit test in src/observe/__tests__/format-cost.test.ts

Expected behavior

import { observe } from 'tealtiger';
import { formatCost } from 'tealtiger/observe';

const client = observe(new OpenAI());
// ... make some calls ...
console.log(formatCost(client.getCost()));
// Output: "$0.0023 (2 requests) | Input: $0.0018 | Output: $0.0005"

Acceptance criteria

• formatCost() exported from tealtiger/observe
• Handles zero-cost case gracefully: "$0.00 (0 requests)"
• Handles pricing gaps: appends " (estimated)" when hasPricingGaps === true
• Unit test with at least 3 cases (zero, normal, pricing gap)
• No external dependencies

Helpful links

• Type definition: src/observe/types.ts → ObserveCostSummary
• Existing formatters: dashboard/web/src/utils/formatters.ts (for reference pattern)