feat: add append_atlan_tags to Batch and save_merging_cm

[mothership-ai]

Session owner: @Pawan-atlan

Summary

• Adds append_atlan_tags parameter to Batch, AsyncBatch, save_merging_cm, and update_merging_cm — enabling incremental Atlan tag operations (add/update/remove) via the bulk API without replacing all existing tags
• Previously, the Batch class only supported replace_atlan_tags (overwrite all or ignore all), making it unusable for high-volume tag sync workflows where specific tags need to be added/removed on 100K+ assets
• The save() method already supported append_atlan_tags, but the Batch class and save_merging_cm/update_merging_cm methods did not pass it through

Context (REQ-717)

A customer running a PyAtlan-based tag sync workflow across ~100K Snowflake assets is experiencing:

1. Performance bottleneck: Each add_atlan_tags()/remove_atlan_tag() call does an IndexSearch + individual save (~0.5-1s per asset)
2. Stability issues: Occasional hangs when the backend is overloaded from sequential requests
3. Unnecessary IndexSearch: _modify_tags() always fetches by qualifiedName even when the customer has the GUID

Recommended approach for the customer (with this fix)

from pyatlan.client.asset import Batch
from pyatlan.model.assets import Table
from pyatlan.model.structs import AtlanTag

batch = Batch(
    client=client,
    max_size=20,
    append_atlan_tags=True,  # NEW parameter
)

for guid, tags_to_add, tags_to_remove in changes:
    asset = Table.ref_by_guid(guid)
    if tags_to_add:
        asset.add_or_update_classifications = [
            AtlanTag(type_name=t) for t in tags_to_add
        ]
    if tags_to_remove:
        asset.remove_classifications = [
            AtlanTag(type_name=t) for t in tags_to_remove
        ]
    batch.add(asset)

batch.flush()

This reduces 100K+ individual API calls (2 per asset) to ~5K bulk API calls (batches of 20), eliminating the IndexSearch overhead entirely.

Test plan

• Verified Batch, AsyncBatch, save_merging_cm, update_merging_cm all accept append_atlan_tags parameter
• Verified flush() passes append_atlan_tags through to save() and save_merging_cm()
• Ruff lint passes on all changed files
• Integration tests should verify tags are appended (not replaced) when append_atlan_tags=True

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