[observe] Add session duration to ObserveCostSummary

[nagasatish007]

Summary

Add sessionDurationMs: number to the ObserveCostSummary type — time elapsed since observe() was called.

What to do

• Record Date.now() when observe() creates the proxy
• Compute duration in getCost() as Date.now() - startTime
• Add sessionDurationMs to the returned ObserveCostSummary object
• Update the type definition
• Add unit test

Expected behavior

const client = observe(new OpenAI());
// ... wait 5 seconds, make some calls ...
const cost = client.getCost();
console.log(cost.sessionDurationMs); // ~5000

Acceptance criteria

• sessionDurationMs field added to ObserveCostSummary interface in src/observe/types.ts
• Value computed correctly as Date.now() - proxyCreationTime
• Unit test verifies duration is > 0 and approximately correct
• Doesn't break existing getCost() consumers (additive change)

Helpful links

• Types: src/observe/types.ts → ObserveCostSummary
• Observe entry point: src/observe/observe.ts
• Cost accumulator: src/observe/cost-accumulator.ts