python(feat): campaigns resource

[Brandon-Shippy]

Description

Adds a campaigns resource to sift_client. A campaign groups the reports from a series of runs, so a whole test program can be reviewed at once.

• CampaignsAPIAsync with the usual get, list_, find, create, update, archive, unarchive, plus get_or_create. Wired up as
  client.campaigns
• CampaignsLowLevelClient wraps CampaignService; the high-level client has zero proto imports
• Pydantic models in sift_types/campaign.py
• add_runs(...) and add_reports(...) grow a campaign without replacing its report list
• report_summaries(...) returns the per-report rule rollup; Campaign.report_summaries() and Campaign.runs read it back from the campaign
• RunCreate.create_default_report so a run generates the report a campaign holds
• Sync wrappers and type stubs registered

Validation

• End to End testing with manual script written to test each new API feature.
• Verified against a local stack, including a multi-run test program collected into one campaign

[github-actions]

Python docs preview: https://sift-stack.github.io/sift/python/pr-795/

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