fix: only inject session tokens into trusted app links - #31208

External app links that use $SESSION_TOKEN now receive a session token only when they open a known Coder IDE handler and point to this deployment.

1. Destination check. The token is filled in only when the link starts with a known handler (<scheme>://coder.coder-remote/open or /openDevContainer, jetbrains://gateway/coder or /com.coder.toolbox, or jetbrains-gateway://connect#type=coder), has exactly one url param that is this deployment's bare origin (dashboard origin or access URL), and the placeholder is the token value. Links with a non-standard shape are rejected. Other links mint no key and render disabled with a tooltip.
2. Sub-agent apps never get a token, like coder open app. Links to this deployment open with an empty token.
3. .devcontainer: drop the now unused placeholder from the Cursor and Windsurf links.
4. Messages: blocked links say "This app's URL isn't trusted to sign you in to Coder." Tooltips point dev container apps to devcontainer.json and template apps to the template administrator.

Behavior changes

• Token links with another handler, a different url, or a non-standard shape are disabled. Registry IDE modules are unaffected.
• Dev container apps using $SESSION_TOKEN open without a token, so the IDE signs in once.
• Templates that set provider "coder" { url = ... } to a different host get their IDE links blocked.

Backports: #31209 (2.38), #31210 (2.37), #31211 (2.36), #31212 (2.34), #31214 (2.29).

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