campaigns add/update: shared bidding-strategy builder foundation

[axisrow]

Parent: #250

Scope

Introduce reusable typed payload building for shared nested BiddingStrategy optional structures without exposing JSON blobs.

CLI Contract

• Use canonical typed CLI flags only.
• Do not add public --json input or blob payload forwarding.
• Keep README/help examples single-line.
• Validate only documented local constraints unless this child issue explicitly says otherwise.

WSDL Audit Routing

Shared Campaign*.BiddingStrategy rows that are prerequisites for subtype-specific child issues.

Acceptance Criteria

• Focused dry-run tests prove the exact Yandex Direct payload shape.
• tests/test_wsdl_parity_gate.py routes the matching optional audit rows to this issue or marks them supported.
• tests/WSDL_OPTIONAL_FIELD_AUDIT.md is updated consistently.
• Relevant CLI/help/docs tests pass.

Out Of Scope

• Live API write tests.
• Unrelated subtype blocks from parent campaigns add/update: complete optional WSDL field parity #250.
• Broad all-optional-fields parity outside this child issue.

[axisrow]

Milestone triage for 0.3.12:

#290 is too large for one patch-sized reviewable PR: current WSDL audit routes 1,939 Campaign*.BiddingStrategy.* missing-followup rows here across Text, Unified, DynamicText, Smart, MobileApp, and CpmBanner add/update branches.

Splitting this epic into post-0.3.12 children:

• campaigns bidding strategies: text/dynamic/unified search branches #346: Text/Dynamic/Unified search branches
• campaigns bidding strategies: text/dynamic/unified network branches #347: Text/Dynamic/Unified network branches
• campaigns bidding strategies: smart campaign branches #348: SmartCampaign search/network branches
• campaigns bidding strategies: mobile app and CPM banner branches #349: MobileAppCampaign and CpmBannerCampaign branches

I am moving #290 out of milestone 0.3.12 as a tracking epic. The remaining 0.3.12 campaign issues #291-#296 will be completed as compact typed-flag PRs for non-BiddingStrategy rows currently routed to those issues.