docs: clarify bundle tests mock network and model downloads

[katebabakova444]

Clarifies that bundle tests run in CI and must not make live network calls, use API keys, or download models.
Updates documentation to explicitly mention mocking:

• HTTP clients
• LLM clients
• embedding/model loaders
• first-run model downloads
  Also notes that real inference belongs in maintainer tests or local/manual runs, not bundle CI gates.

Related issue

Closes #170

[rosspeili]

Thanks @katebabakova444, this closes #170 cleanly.

LGTM once CI passes. Suggested polish: blank line before ### Framework test in TESTING.md, and a one-line [Unreleased] Documentation entry in CHANGELOG if you want full PR checklist parity.

Nice, focused doc PR, appreciate it, and welcome <3

[katebabakova444]

Thanks! Added the TESTING.md spacing polish and Unreleased changelog entry.

[rosspeili]

Merged in main. Thank you for this clean take. Feel free to explore more good first issues, or suggest issues or new skills if you're using the framework. <3