Allow protocol era to be set on an ad-hoc --server-url launch (not just via `mcp.json`)

[kraenhansen]

Which client?

All / shared core

The problem

protocolEra is only settable as a per-server field in an mcp.json entry. An ad-hoc invocation (--server-url + --header) always gets the default legacy era, in all three clients.

We launch the inspector from a wrapper script that mints a short-lived OAuth token and spawns:

mcp-inspector --transport http --server-url "$URL" --header "Authorization: Bearer $TOKEN"

To exercise the same server under a non-legacy era we would have to write a config file per launch, which means writing the bearer token to disk.

Solution you have in mind (optional)

A --protocol-era <legacy|auto|modern> flag that feeds settings.protocolEra on the ad-hoc path, the way --connect-timeout feeds settings.connectionTimeout today.

Alternatives or workarounds you have tried

Generating a temporary mcp.json per launch (puts a short-lived credential on disk). In web, switching Protocol Era by hand in Server Settings after every launch; CLI and TUI have no equivalent.

Before you submit

• I searched existing issues and this is not a duplicate.
• This is a request for the Inspector itself, not for the MCP specification or an SDK.

[cliffhall]

Triage: Priority Low (total 5)

• Severity 3 — a real capability is missing on the ad-hoc path; the workaround (a generated mcp.json) works but writes a short-lived credential to disk, and CLI/TUI have no in-session equivalent
• Urgency 2 — wanted eventually; nothing is blocked on it
• Bonuses: none (no bug label, no milestone, unassigned, not a sub-issue, no reporter priority set)

Labels applied at triage: v2, enhancement.

Board: #28, Status Incoming (awaiting maintainer review — no milestone yet).