Fix heartbeat task-slot/poller info reporting 0 with MetricBuffer

[Mahnoor-Zaffar]

What & Why

Fixes #1817. When a worker uses TelemetryConfig(metrics=MetricBuffer(...)), every labelled worker-heartbeat metric (task-slot and poller counts) was reported to the server as 0, hiding them from the Workers UI.

The bug is in Core: heartbeat slot/poller gauges are written via an in-memory tap that resolves labels from metric attributes, but the buffered attributes handle couldn't be read core-side. This PR bumps the sdk-core submodule to a commit that retains the key-value pairs core-side so label resolution works without touching the lang-held buffer.

The actual Core fix lives in temporalio/sdk-rust#1593.

Changes

• Bump temporalio/bridge/sdk-core submodule to 7d95a31e (buffered heartbeat metric label fix).
• CHANGELOG entry under [Unreleased].

Validation

• poe build-develop (rebuilds the Rust bridge against the new submodule).
• poe lint, poe test -s -k <metric pattern>, poe bridge-lint.
• The public repro repo (https://github.com/bmcqueen-tmprl/temporal-py-heartbeat-slot-repro) was used to confirm slot/poller counts are no longer 0 with MetricBuffer.

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