### Check for existing issues

- [x] I have searched the existing issues and checked that my issue is not a duplicate.

### What happened?

**What happened:** When configuring an MCP server in the Admin UI, setting a **Static Header** value to `os.environ/<ENV_VAR_NAME>` does not resolve to the actual environment variable at runtime. The literal string `os.environ/<ENV_VAR_NAME>` is stored in the DB and sent on outbound MCP requests, so auth fails.

Pasting the real secret value manually in the same Static Headers field works.

**Expected:** `os.environ/<VAR>` in MCP `static_headers` should resolve via `get_secret()` / `os.getenv()`, consistent with other proxy config fields and YAML `mcp_servers` config.

**Root cause (for maintainers):** `ProxyConfig._check_for_os_environ_vars()` resolves `os.environ/` references for YAML config at load time, but `build_mcp_server_from_table()` deserializes `static_headers` from the DB without resolving them. The Test Connection path also passes `request.static_headers` through unchanged.

### Steps to Reproduce

1. Set a secret in the proxy process environment, e.g.: export MY_SECRET="some-value"

2. Start LiteLLM proxy with DB enabled.…