Make ParameterMapper generic on Content type

[Kamilbenkirane]

Problem

ParameterMapper.parse_output() in src/celeste/parameters.py uses Any for its content parameter and return type. This was introduced as a pragmatic fix in #142 to bridge the type boundary between the generic Content type parameter on ModalityClient and the text-specific parse_output method.

While Any is accurate for the base implementation (it's an identity function), it loses type safety — callers and overrides get no static type checking on the content flowing through parse_output.

Root cause

ParameterMapper is not generic. It was originally designed with TextContent hardcoded in parse_output, because structured output parsing only applies to text. But now that _transform_output() is called at the base _predict() pipeline level (for all modalities), the parameter system needs to be content-type-agnostic.

Proposed fix

Make ParameterMapper generic on a Content type parameter, following the existing codebase convention (PEP 695 syntax, semantic names):

class ParameterMapper[Content](ABC):
    def parse_output(self, content: Content, value: object | None) -> Content:
        return content

Then propagate through the chain:

• FieldMapper[Content](ParameterMapper[Content])
• parameter_mappers() -> list[ParameterMapper[Content]] on ModalityClient
• _transform_output(content: Content, ...) -> Content — no Any needed

Affected files

• src/celeste/parameters.py — make ParameterMapper and FieldMapper generic
• src/celeste/client.py — update parameter_mappers() return type
• 70+ ParameterMapper subclasses — add type parameter
• 40+ FieldMapper subclasses — add type parameter
• 7 parse_output overrides — change TextContent to Content
• All parameter_mappers() implementations — update return type

Impact

• Replaces Any patch from Move _transform_output from provider _parse_content to base _predict pipeline #142 with proper end-to-end type safety
• Makes the parameter system truly modality-agnostic at the type level
• No runtime behavior changes — purely a typing improvement