Define a minimal OperationParams SPI feeding the context chain

[OmarAlJarrah]

Problem

Thin generated services need a way to project an operation's inputs into headers/query/path/body that feeds our context chain.

Proposed change

Define a small OperationParams SPI exposing header/query/path/body projections that populate DispatchContext/RequestContext and our Headers/QueryParams. Public and explicit-API-clean (no underscore pseudo-privacy).

Acceptance

• OperationParams SPI
• Feeds the existing context chain
• apiDump

Depends on the QueryParams work. Precondition for thin services + typed paging.

Dependencies

• Blocked by Build a first-class QueryParams multimap and remove the QueryParam stub #28 — QueryParams multimap
• Blocks Generate two-tier raw/cooked service methods #55 — two-tier services
• Blocks Typed page classes that rebuild typed params, not URL strings #56 — typed page classes

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Priority: high · Effort: small