[air] Add GPU_8xB300 support to DABs

[caroline-db]

Changes

• Add GPU_8xB300 to the Bundle schema, enum validation, and generated PyDAB model for AI Runtime tasks.
• Enable experimental air convert-to-dabs to emit B300 job configurations using environment version 6.

Why

Complete DAB support for the B300 functionality introduced in #6649. The development-stage SDK enum does not yet include B300, so this adds a narrow CLI-owned extension until the SDK publishes it.

Jobs remains authoritative for accelerator/environment compatibility, including the B300 minimum version. This PR does not add a DAB cross-resource minimum-version validator; a follow-up may incorporate that policy into --dry-run once its final behavior is defined.

Tests

• Passed full Go lint, 10,024 unit tests, and 5,134 acceptance tests.
• Regenerated Bundle schema, enum validation, and PyDAB outputs; all 38 PyDAB generator tests passed with no unrelated drift.
• Full PyDAB test/lint/docs and direct-engine regeneration were blocked locally by PyPI DNS while fetching build dependencies.

This PR was written with Codex.