fix: stabilize LocalToken so client configs never go stale (401)

[IvanMurzak]

Summary

• BUG-B (mcp-authorize i2, mcp-authorize i2: stabilize LocalToken and rewrite client configs on token change (fix 401) #897): in Custom + token mode the client .mcp.json ended up with a DIFFERENT token than the running local server → Claude Code 401.
• Root cause / regen trigger identified: UnityConnectionConfig.SetDefault() assigns the freshly-generated local server token through the mode-routed Token setter after setting ConnectionMode = Cloud. So Token = GenerateToken() routed the local secret into CloudToken and left LocalToken unseeded. LocalToken initialization then fell entirely to the generate-if-empty fallback in GetOrCreateConfig, which let the persisted local token drift / regenerate and orphan the already-written client .mcp.json (stale Bearer → 401).
• Fix: seed LocalToken directly in SetDefault() (GenerateToken() produces a local server secret; CloudToken is populated by Cloud sign-in, so it now correctly stays null). A persisted LocalToken is stable across construction, mode re-apply, and a serialize/deserialize (domain reload / save-load) round-trip, so the server launch token (McpServerManager → UnityMcpPluginEditor.Token) and the client-config Bearer (AgentConfiguratorSettingsFactory → UnityMcpPluginEditor.Token) stay in lockstep.
• Token change (New button) → reconfigure: left unchanged. That path already restarts the server and calls InvalidateAndReloadAgentUI() → re-evaluates _configurator.GetStatus(...) == ReconfigureNeeded and shows the "Reconfiguration Required" alert (driven by sibling task i1's token-aware validator). No new plumbing needed here.
• No version bump / no McpPlugin re-pin (client-side-only fix; coordinated bump + release is task i6).

Test plan

• New LocalTokenStabilityTests (EditMode) — 5 tests, all pass: SetDefault seeds LocalToken not CloudToken; LocalToken stable across mode re-apply; survives save/load round-trip; round-trip leaves it non-empty (generate-if-empty stays a no-op); serializes under the token JSON field.
• Full Unity EditMode suite (2022.3.62f3) — 0 failures across all runnable fixtures (chunked). One fixture, TestsRunDirtySceneTests, cannot run through the MCP tests-run tool (it mutates Tool_Tests' active-run bookkeeping, self-interfering with the tool's completion detection) and is validated by CI's direct Unity Test Runner instead — unrelated to this diff.
• Unity PlayMode suite — 0 failures.

Closes #897