improvement: remove waits before fetching activities (#8144)

## What changed?
optimize the batch operation processing in `BatchActivity` and
`BatchActivityWithProtobuf` by removing the need to wait for entire
pages to complete before fetching the next page.

- Implemented proactive page fetching once a worker becomes available
- common `processWorkflowsWithProactiveFetching` function to reduce code
duplication

## Why?
The previous implementation had workers wait for entire pages to
complete. This optimization improves resource utilization. The
refactoring also eliminates duplicated functions in the `BatchParams`
struct and `BatchOperation` protobuf.

Addresses issue #8098.

## How did you test it?
- [x] built
- [x] run locally and tested manually
- [x] covered by existing tests
- [ ] added new unit test(s)
- [ ] added new functional test(s)

The changes maintain backward compatibility.

## Potential risks
While this change improves performance, it does modify the concurrency
model of batch processing:

1. **Timing changes**: The optimization changes when pages are fetched
relative to task completion, which could expose edge cases in error
handling or heartbeat timing
2. **Memory usage**: Pages may be fetched earlier, potentially
increasing peak memory usage if the next page is large
3. **Rate limiting interaction**: The more aggressive task scheduling
could interact differently with rate limiting, though the same
per-worker limits are maintained
4. **Heartbeat behavior**: heartbeats track the progress of an entire
page and are applied after an entire page finishes

The changes preserve all existing error handling, retry logic, and rate
limiting behavior, but the different execution timing could surface
previously hidden race conditions.

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Co-authored-by: Roey Berman <[email protected]>

Author: spkane31