refactor(sharddistributor): remove migration passthrough modes from executor client

[eleonoradgr]

What changed?
Removed the LOCAL_PASSTHROUGH_SHADOW and DISTRIBUTED_PASSTHROUGH migration modes from the executor client, along with the AssignShardsFromLocalLogic and
RemoveShardsFromLocalLogic interface methods and their implementations. Also removed the compareAssignments/emitMetricsConvergence helpers and the associated
convergence/divergence metrics. Relates to #6862.

Why?
These migration modes and the external-assignment methods were part of a non used migration path for onboarding namespaces to shard distributor. The
LOCAL_PASSTHROUGH_SHADOW and DISTRIBUTED_PASSTHROUGH modes acted as intermediate steps that are no longer needed since we rely on fully onboarded mode and percentage base onboarding(ONBOARDED mode).
Keeping dead code around increases cognitive load and leaves error-prone paths.

How did you test it?
make pr GEN_DIR=service/sharddistributor/client/executorclient

Package unit tests (race detector enabled)

go test -race -count=1 ./service/sharddistributor/client/executorclient/...
Result: ok github.com/uber/cadence/service/sharddistributor/client/executorclient 3.714s

Full build check

make build

Potential risks
None — the removed methods were only called from code already cleaned up in prior PRs. No API/IDL, schema, or feature flag changes.

Release notes
N/A — internal change.

Documentation Changes
N/A

[gitar-bot]

Code Review ✅ Approved

Removes migration passthrough modes from the executor client, simplifying the shard distributor implementation. No issues found.

Rules ✅ All requirements met

Repository Rules

✅ No CI failures detected: No CI failures were detected in the PR.

✅ PR has description and tests: PR includes a detailed description of what changed, why, and how it was tested including unit tests and build checks.

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