fix(composer): preserve draft when picking provider commands

[kvnloo]

Stacked on #13777. Fixes the unresolved web/mobile composer review finding where selecting a no-argument provider command could delete surrounding draft text.

Behavior

A no-argument provider command auto-runs only when the selected slash-command trigger is the entire non-whitespace draft.

Examples:

• /copy → auto-run
• /copy → auto-run
• /copy keep this draft → stays editable; surrounding text is preserved
• keep /copy → stays editable

Web and mobile use the same pure trigger-only predicate.

Tests

Adds focused shared coverage for standalone vs mixed-content trigger selection.

No provider/session/runtime behavior changes.

[macroscopeapp]

Approvability

Verdict: Approved at f733096

Macroscope's review found this PR approvable — This is a small composer bug fix that prevents no-argument provider commands from auto-running when surrounding draft text exists, while preserving the existing standalone-command behavior. The shared predicate is focused and tested, and the runtime impact is limited to web and mobile composer selection.

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[juliusmarminge]

Note

This comment is posted by Julius' dot

This changes when web and mobile composer selections execute a command, but the PR only describes added assertions and supplies no observed test result or UI evidence. Please report the focused test command and result, and add before/after screenshots plus a short recording showing standalone commands still run while mixed drafts stay editable on both clients. The verification rule requires evidence for the changed interaction. Closing pending that evidence; add it and request reconsideration.