Fix combo paths with embedded colons

[bneradt]

Combo handler query parameters use a colon to select the bucket prefix,
but asset paths can also contain colons. Requests with an embedded path
colon were split at the last colon, producing the wrong internal fetch
URL and failing instead of fetching the requested object.

This keeps the first colon as the combo delimiter and leaves later
colons in the asset path. This also adds an AuTest case with a neutral
asset name so the parser regression is covered.

[Copilot]

Pull request overview

This PR fixes combo_handler query parsing when an asset path contains embedded colons. Previously, the parser treated the last colon in a query parameter as the bucket/path delimiter, which broke internal fetch URLs for assets that legitimately include : in their path.

Changes:

• Update combo_handler query parsing to treat only the first colon in each parameter as the bucket delimiter.
• Add an AuTest origin asset whose path contains a colon and a corresponding request to cover the regression.
• Add remap coverage for the new /s/ bucket used by the regression test.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
plugins/esi/combo_handler.cc	Adjusts colon detection so only the first colon per query parameter is used as the bucket/path separator.
tests/gold_tests/pluginTest/combo_handler/combo_handler.test.py	Adds an origin object and AuTest run validating that combo requests succeed when the asset path contains an embedded colon.

[bneradt]

Basically: make sure colon_pos hasn't been set yet before setting it in case there is a : in the path too.