fix(branch): add MERGING to BranchStatus enum (infrahub#9293)

[PhillSimonds]

Summary

• Adds MERGING to infrahub_sdk.branch.BranchStatus so the SDK enum matches the server-side backend/infrahub/core/branch/enums.py enum (which gained MERGING in Infrahub 1.9.3 via greater rollback coverage of merge logic infrahub#9116).
• Without this, client.branch.all() raises pydantic_core.ValidationError for any branch returned with status="MERGING", which crashloops infrahub-task-worker on every sync_remote_repositories run if a merge task dies mid-flight and leaves a branch stranded in MERGING.
• Fixes bug: infrahub-task-worker crashloops with BranchData ValidationError when a branch is in MERGING status (SDK enum missing MERGING) #1037 (migrated from bug: infrahub-task-worker crashloops with BranchData ValidationError when a branch is in MERGING status (SDK enum missing MERGING) infrahub#9293).

This patches the SDK side of the bug only — the server-side robustness improvements (guaranteeing a branch never stays in MERGING, or making BranchManager.all() tolerant of one malformed entry) are out of scope here and would land in opsmill/infrahub.

Reproduction (live, against Infrahub 1.9.5)

# Create a branch
curl -s -X POST http://localhost:8000/graphql/main -H "Authorization: Bearer $TOKEN" \
  -d '{"query":"mutation { BranchCreate(data: {name: \"repro-9293\", sync_with_git: false}) { ok object { id name status } } }"}'

# Strand it in MERGING
docker exec infrahub-database-1 cypher-shell -u neo4j -p admin \
  "MATCH (b:Branch {name: 'repro-9293'}) SET b.status = 'MERGING' RETURN b.status"

# Before this fix — inside the task worker:
docker exec infrahub-task-worker-1 python -c "
import asyncio
from infrahub_sdk import InfrahubClient, Config
asyncio.run(InfrahubClient(address='http://server:8000',
    config=Config(username='admin', password='infrahub')).branch.all())
"
# -> pydantic_core._pydantic_core.ValidationError: 1 validation error for BranchData
#    status
#      Input should be 'OPEN', 'NEED_REBASE', 'NEED_UPGRADE_REBASE', 'DELETING' or 'MERGED'
#      [type=enum, input_value='MERGING', input_type=str]

# After this fix:
# -> OK: client.branch.all() returned 2 branches
#      - main: status=OPEN
#      - repro-9293: status=MERGING

Test plan

• pytest tests/unit/sdk/test_branch.py — 12 passed.
• Live end-to-end repro against Infrahub 1.9.5 (infrahub-task-worker:1.9.5 image), patched in-container via docker cp, client.branch.all() now returns the MERGING branch.
• Cross-enum coverage test will be added in opsmill/infrahub as part of the PR that bumps the SDK commit pin (per review feedback) — that side can iterate the actual server-side BranchStatus enum.

Follow-ups (separate PRs / not in this changeset)

• opsmill/infrahub: bump python_sdk submodule pin once this is merged, target a 1.9.6 patch release; add the parametrized enum-coverage test there.
• opsmill/infrahub: wrap _do_merge_branch body in try/finally so a worker death between the MERGING transition and MERGED/rollback can't permanently strand a branch.
• opsmill/infrahub: optionally have BranchManager.all() skip+log malformed entries rather than failing the whole collection.

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Codecov Report

✅ All modified and coverable lines are covered by tests.

@@           Coverage Diff           @@
##           stable    #1036   +/-   ##
=======================================
  Coverage   81.54%   81.54%           
=======================================
  Files         134      134           
  Lines       11467    11468    +1     
  Branches     1735     1735           
=======================================
+ Hits         9351     9352    +1     
  Misses       1569     1569           
  Partials      547      547           

Flag	Coverage Δ
integration-tests	41.71% <0.00%> (-0.01%)	⬇️
python-3.10	54.56% <0.00%> (-0.03%)	⬇️
python-3.11	54.56% <0.00%> (-0.03%)	⬇️
python-3.12	54.58% <0.00%> (-0.01%)	⬇️
python-3.13	54.58% <0.00%> (-0.01%)	⬇️
python-3.14	54.56% <0.00%> (-0.01%)	⬇️
python-filler-3.12	22.67% <100.00%> (+<0.01%)	⬆️

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Files with missing lines	Coverage Δ
infrahub_sdk/branch.py	68.75% <100.00%> (+0.21%)	⬆️

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