Simplify E2B template build onboarding

[RyanLee-Dev]

Template builds previously required operators to transfer connection and immutable build selectors into Web by hand. The builder now accepts explicit API URL and domain settings and emits a validated, nonsecret manifest that Web can import. Changing the destination clears the entered key; Core continues to validate the selected build.

The setup flow also provides template-building guidance, exact build entry, and separate template/build discovery errors. Python and TypeScript validate the handoff against shared fixtures. The owning builder and Web documentation describe the workflow.

Validation:

• 193 Python deployment/provider tests passed with the pinned E2B 2.51.0 SDK.
• 496 Web tests, typecheck, and production build passed.
• All 10 relevant browser cases passed across focused runs, including five new import cases and existing E2B flows; English and Chinese screenshots inspected.
• Naming/documentation checks and independent blind review passed with no actionable findings.
• Live official E2B template listing succeeded (empty custom-template catalog). The user's SDK create/echo example succeeded and the sandbox was deleted.

Draft: full OAC Runtime template deployment and third-party cloud acceptance remain unverified. The supplied credential was confirmed to belong to official E2B; the earlier SandBase 401 therefore provides no SandBase compatibility evidence. No credentials are included in the commit or manifest.

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