feat(backend): switch runtime MLMD tracking to task APIs

[HumairAK]

Description of your changes:

This PR ports the runtime slice of the MLMD removal work onto the current mlmd-removal base and includes the follow-up compiler/runtime fixes needed to keep that path stable in API and E2E coverage.

• switch backend runtime tracking from MLMD-backed execution state to KFP task and artifact APIs, including the cached-task lookup surface and regenerated v2beta1 clients
• remove the remaining runtime MLMD-dependent flow and tighten run-scoped task updates around iteration identity, final-status lookup, importer matching, archived run preservation, and artifact task links
• update compiler/runtime integration to preserve loop context through nested DAGs, keep shared container executor iteration handling valid, and regenerate runtime-focused tests and compiled workflow goldens

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