refactor(plugin): decode RPC values through conversion traits

The R-table dispatchers hand-rolled argument decoding and return encoding per
method, spread over 24 helpers. `FromPluginArg` and `ToPluginValue` replace
them: blanket impls for `Option<T>`, `Vec<T>` and `IndexMap<String, T>` compose
over a handful of leaf types, so a composite like `Vec<IndexMap<String, String>>`
needs no helper of its own, and the lossy UTF-8 conversion of a PHP string lives
in one impl instead of a dozen call sites.

Argument errors now read `{method} expects {expected} at position {position}`
rather than a message written per call site. `arg_or` takes its default for an
explicit null too, not only for an omitted argument; every parameter it serves
is declared non-nullable in the proxy stubs, so PHP never passes one.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

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