Add Search campaign expansion, asset tools, and CPC-cap support

[sblOWPCKCR]

• add draft_ad_group and update_ad_group for existing campaigns
• extend draft_campaign/update_campaign with Search partners and display expansion settings
• support max_cpc for MANUAL_CPC ad-group bids and TARGET_SPEND (Maximize Clicks) CPC caps
• add draft_callouts, draft_structured_snippets, and draft_image_assets
• create shared campaign-asset mutation helper for campaign-level assets
• fix image asset uploads by setting required asset names and preserving compatibility with older pending plans
• add write-layer tests covering validation, field masks, campaign settings, asset creation, and image upload regressions

[kLOsk]

Merged manually after rebasing on main (which now includes PRs #2, #3, #6).

Changes during rebase:

• Removed duplicate draft_ad_group — PR Add draft_ad_group tool for creating ad groups in existing campaigns #6's version (already merged) has better preflight checks (campaign type validation, duplicate name detection, broad match safety) and keyword support. Kept that one.
• Kept update_ad_group from this PR — new functionality not covered by PR Add draft_ad_group tool for creating ad groups in existing campaigns #6.
• Kept all other features: campaign expansion settings, asset tools (callouts, structured snippets, image assets), CPC cap support, structured error handling, _apply_campaign_assets refactor.
• Resolved parameter conflicts: merged PR Fix hardcoded EUR currency — auto-detect from Google Ads account #2's customer_id currency parameter with this PR's expansion parameters in _validate_campaign.
• Removed 3 tests that tested the dropped draft_ad_group implementation. All 77 remaining tests pass.

Thanks for the substantial contribution @sblOWPCKCR!