Per-server namespaced inputs for bundle config — prevent key collisions when MCP servers share input names

[mabry1985]

Problem

The #2041 create-time picker pipeline carries operator input values as a flat inputs: {key: value} map end to end — the ${key} placeholder contract from config/mcp-catalog.json/#2011, shipped on POST /api/fleet in #2125 and consumed by the picker in #2127. Consequence (QA panel finding on #2127, confirmed; adjudicated merge-then-followup): two MCP servers in one bundle that declare the same input key (e.g. both need a token) collide — the picker's fieldId is origin:key (no server), and even a namespaced UI collapses back to flat at submit, so one value fills both servers' templates. No bundle we ship today declares two servers sharing an input key, but nothing prevents one.

Proposed direction

• Accept inputs keyed "<server>:<key>" with bare-key fallback on POST /api/fleet (backward compatible).
• resolve_bundle_mcp_item gains per-item override resolution: prefer values["<item.name>:<key>"], then values[key], then env/default.
• Picker: fieldId becomes origin:server:key; splitConfigValues emits namespaced keys only when a collision exists (keeps the common case clean).

Acceptance

• A bundle with two MCP servers both declaring input token resolves distinct operator-supplied values per server, end to end (picker → route → resolver).
• Single-server bundles and bare-key callers behave exactly as today (regression tests on the feat: Backend — accept operator inputs + secrets at fleet create time #2125/feat: Frontend — input picker in NewAgentPanel + preview dialog enrichment #2127 suites).

Refs

#2041, #2125 (flat contract), #2127 (the finding), config/mcp-catalog.json input contract.