Release v2.13.0: avatar provider defaults and typed LemonSlice options

[digitallysavvy]

Provider-branded avatar constructors currently require an explicit endpoint, and LemonSlice-specific image and ratio fields must be supplied through additional params. This release adds thin GenericAvatar wrappers with Tavus, Protoface, and LemonSlice endpoint defaults; callers can still override the URL.

LemonSlice defaults the avatar ID to lemonslice and exposes typed agent image URL, agent ID, base64 image, and aspect ratio options. They serialize directly in avatar.params with explicit typed values taking precedence over matching additional params. Validation checks the effective merged configuration: exactly one nonempty selector, and aspect ratios 2x3, 9x16, or 1x1. Existing valid additional-params configurations remain supported, and caller inputs are not mutated.

GenericAvatar serialization, the generic wire vendor, and AgentKit session/token handling are reused. GenericAvatar itself still requires its URL. Branded providers are now wrappers rather than exact aliases; LemonSlice selector validation is mandatory. This compatibility change is documented in the guide, reference, and changelog.

Prepares v2.13.0 with changelog entries dated 2026-10-09 and updated primary/compatibility package versions, compatibility dependency requirements, and runtime SDK version metadata. Preview SDK copies remain untouched. No release tag or publishing is performed by this PR.

Validation:

• Custom suite: 445 passed, 1 skipped; provider tests rerun after test-fixture type fix: 51 passed.
• Full mypy: 359 source files passed; changed-file Ruff checks passed.
• Primary and compatibility wheel/sdist builds, release checks, and git diff checks passed.