feat: add list_profiles and switch_profile tools

[akmukherjee]

Adds two tools for switching between Prefect workspaces in stdio transport (e.g. Claude Desktop):

list_profiles — lists available profiles from ~/.prefect/profiles.toml
switch_profile — stores the selected profile in session state; the middleware then wraps subsequent tool calls in use_profile() so they use that profile's credentials
No new settings or dependencies. Falls back to the default active profile if switch_profile has not been called.

[zzstoatzz]

hi @akmukherjee thank you for the PR, sorry for the sluggish response, this is still on my radar. there are some details i want to think about / talk about with the team but this is generally looking good here and i'll come back for a thorough review as soon as possible