fix(pmax): make create_pmax_campaign work + add asset group linking and signals

[trade74]

While building a real Performance Max campaign through this server I hit three blockers. Two are bugs that make create_pmax_campaign unusable; one is a missing capability without which a PMax campaign cannot serve at all.

All of it is verified end to end against a live Google Ads account (v23), not just against the test suite.

Fixed: create_pmax_campaign could not create any campaign

Every call built a plausible-looking plan and then failed at confirm_and_apply with the server's generic Request contains an invalid argument. Two independent causes:

Google error	Cause
BIDDING_STRATEGY_TYPE_INCOMPATIBLE_WITH_SHARED_BUDGET	campaign_budget.explicitly_shared defaults to true server-side, and PMax rejects a shared budget. draft_campaign got this fix in 5670d2d; the PMax path was missed.
fieldError=REQUIRED on contains_eu_political_advertising	Required on every campaign create since the EU political ads regulation. Omitting it fails the whole mutate.

Added: link_asset_to_asset_group

upload_image_asset only ever created the asset — nothing attached it, so uploaded images sat unused in the account's asset library. A PMax asset group without a MARKETING_IMAGE, SQUARE_MARKETING_IMAGE and LOGO reports NOT_ELIGIBLE and never serves.

The field type is validated client-side against the 11 types PMax accepts, because the API error for a bad one is opaque.

Added: add_asset_group_signal

PMax does not take audiences as campaign criteria. add_audience_targeting writes a campaignCriterion userList, which the API rejects for a PMax campaign — so the server had no working way to set the single most important PMax targeting input. Signals belong on asset_group_signal.

Takes search themes, audience IDs, or both, and enforces the 80-character search theme limit before the request goes out.

Added: scripts/gads_mutate.py

None of the above was diagnosable from the server's output, which collapses Google's nested error tree to one generic string. This replays a mutate payload and prints the full tree, with --check for validateOnly. It is what surfaced all three problems.

Testing

• 14 new unit tests (293 total, all passing); cargo clippy -D warnings and cargo fmt --check clean.
• Verified live: a PMax campaign with an asset group, 26 text assets, 9 image assets across all five image field types, and 9 asset group signals now apply cleanly.

Two behaviours worth recording for anyone debugging PMax here, both found the hard way:

• validateOnly does not resolve negative temp resource names when it counts an asset group's minimum assets, so a single atomic create reports false NOT_ENOUGH_HEADLINE_ASSET errors. Pre-creating the text assets and referencing real IDs validates clean.
• With Brand Guidelines enabled on an account, BUSINESS_NAME, LOGO and LANDSCAPE_LOGO must be linked as CampaignAssets and are rejected as asset-group assets.

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