fix(studio): recover ambiguous agent updates

[zyn080302]

Summary

• preserve accepted Agent deployments as a non-terminal status when POST/SSE transport is ambiguous instead of presenting a false terminal failure or retry
• allow explicitly confirmed stored-credential reuse for an unnamed MCP only when the same Agent graph path, MCP slot, and credential reference are proven
• align MCP URL guidance with the actual /mcp suffix validation
• add the reproduced customer bad cases to the Studio release evaluation gate

Validation

• frontend: 1048/1048; focused recovery: 54/54
• Python related suites: 55/55
• Studio release gate: Python 74/74 with 96% Sidecar coverage; frontend 22/22 with 94.44% branch coverage
• i18n, Ruff, Pyright, pre-commit, secret scan, two-target production build, and packaged WebUI asset validation passed