Make the two session accessors return what they hold

[blaipr]

The bug

Two dormant type mismatches in Session, both found while covering it:

• getAccountColor(): string returned whatever setAccountColor(array $color) had stored — always an array. The first read after a legitimate write would have raised a TypeError on the way out. SessionContext declared the same mismatch.
• getTheme(): string had no default on its read, so a session that had not been given a theme returned null from a method declared to return a string.

Neither is called anywhere in src/ today, which is the only reason it went unnoticed — and exactly why it was worth fixing before somebody used one.

The fix

Both now return what they always held: an array of colours keyed by account id, and a string that is empty before a theme is chosen.

Testing

A round trip for each, including what a session that has never been given either returns — which is where both used to raise.

Doc, folded in

• Coverage is now 96.6% (21648/22402).
• The pcov note gains the thing that made a file look like a gap when it is at 100%: the two suites disagree about which lines are statements in nine files out of 995 (31 statements; Application/Account/Services/AccountSearch.php worst, 48 vs 61). A merge that treats absent from one report as uncovered overstates them, so a per-file gap is worth confirming with a focused run before anyone is sent to fix it.