fix(lsp): refresh client-dependent runtime state

[mehalter]

Configured autocommands

• Resolve a currently attached matching client when each callback runs instead of retaining the client that created the autocommand.
• Scope client lookup to the callback buffer and preserve the autocommand through temporary client gaps.

Signature help

• Rebuild trigger and retrigger unions when clients attach, detach, register capabilities, or unregister capabilities.
• Include applicable dynamic registrations alongside static server capabilities.
• Match Neovim 0.11 document selectors by language, URI scheme, and glob pattern while using the native registration lookup on Neovim 0.12.
• Mark the exact Neovim 0.11 fallback for removal when support is dropped.

Validation

• Passed StyLua, Selene, typos, Lua compilation, and diff checks.
• Passed focused attach, detach, reattach, registration, selector, and cleanup regressions on Neovim 0.11.4 and 0.12.4.