Add a structured outputs lesson to anthropic_api_fundamentals (#161)

[NishchayMahor]

Refs #161.

The Skilljar "Claude with the Anthropic API" course teaches structured JSON extraction with prefill + `stop_sequences`. Structured outputs (`output_config.format` with a JSON schema) is now the purpose-built solution — constrained decoding means the parse step can't fail and no retry loop is needed — and is explicitly incompatible with assistant prefill, so the two paths don't fit cleanly in one lesson.

This PR doesn't touch Skilljar (that's behind the platform), but it fills the gap in the open-source course material:

• New `anthropic_api_fundamentals/07_structured_outputs.ipynb` walks through the contact-card extraction case end to end, shows the legacy prefill+stop pattern next to it for comparison, and ends with a multi-meeting extraction exercise.
• A small table covers when to reach for structured outputs vs. prefill+stop vs. tool use.
• README updated with the link.

Happy to fold the same content into an updated Skilljar script if that's useful — let me know what the cadence is for keeping the platform version in sync.

What's in the notebook

Cell	Content
1	Goals + motivation for structured outputs
2	SDK / client setup (same import block the other lessons use)
3	Contact-card schema
4	`messages.create(... output_config={"format": {"type": "json_schema", "schema": ...}})`
5	Note on prefill incompatibility
6	Legacy prefill + `stop_sequences` for comparison
7	Decision table (which technique for which case)
8	Exercise: extract a list of meetings from a digest

Cells use `claude-sonnet-4-6` to match the GA list from the structured outputs reference.

[debbie-shapiro]

I would also add that in order to run through the exercise as designed, it requires an earlier model (sonnet-4-0) which now gives a deprecation message and will no longer be available as of June 14th. The newer models don't work for this part of the course at all.

[NishchayMahor]

Good flag. The new lesson here only uses claude-sonnet-4-6, which is GA for structured outputs, so this notebook itself won't hit the June 14 sonnet-4-0 deprecation.

The exercises still pinned to sonnet-4-0 are really the broader hardcoded-model issue from #71 rather than anything this PR touches. Happy to take that on as a follow-up — sweep the notebooks and bump the deprecated pins — if you point me at which lessons are in scope.