fix(chromium-tabs): say why a tab cannot be opened in an Electron app

[filip131311]

Fixes #634.

before:  [Tool:chromium-tabs] Not supported

after:   [Tool:chromium-tabs] Could not open a tab: Not supported. An Electron app has no
         browser-level target creation, so a tab cannot be made from outside it — have the app open
         one (e.g. `window.open()` through `debugger-evaluate`) and it will show up in
         `chromium-tabs list`. The list, select and close actions are unaffected.

The old text is Electron's own CDP answer to Target.createTarget, passed through untouched. The
limitation is real, but the message names neither the cause nor a way forward, and reads as though the
tool were unavailable — when only this one action is.

Two things I checked before writing the message

• The workaround actually works. On the Electron testbed I ran window.open('about:blank') via
  debugger-evaluate and the new window appeared in chromium-tabs list as t2. I didn't want to
  ship advice I hadn't run.
• The rest of the tool really is fine — list returns correctly both before and after.

Why the cause is kept rather than matched on

Target.createTarget can fail for reasons other than Electron's missing browser shell — a dropped
connection, or a real Chrome refusing the url. The CDP client keeps only the error's message and
discards its numeric code, so branching would mean matching on the string "Not supported", which
breaks the moment Electron rephrases it.

Instead any failure is wrapped, with the original message embedded. The explanation is worded as a
property of Electron rather than a claim about this failure, so it stays accurate when the cause is
something else — and the underlying error is still right there to read.

Verification

Live against the Electron testbed (Electron 42 / Chrome 148): the message above is the real output,
and list still works alongside it. tool-server 3087 passed, plus a new test asserting the message
keeps the original cause and carries the reason, the workaround, and the note that the other actions
are unaffected.

🤖 Generated with Claude Code