fix(session): ignore malformed model costs

[nexxeln]

Summary

• treat malformed or non-finite model cost fields as zero during usage accounting
• preserve valid pricing components and existing finite-price behavior
• add a regression test for provider-supplied object values

Closes#43098

Testing

• bun test test/session/compaction.test.ts --test-name-pattern "SessionNs.getUsage"
• bun typecheck in packages/opencode
• full workspace typecheck from the pre-push hook

[macurandb]

Question rather than a bug report — is the numeric-string case intentional?

finite() takes a number, so a cost field that arrives as a numeric string now prices at 0, where ?? 0 used to hand it straight to decimal.js, which parses it fine:

cost.input	before	after
"3"	3	0
"1e3"	1000	0
{}	threw	0 ✅

If the answer is "a string violates the declared Schema.Finite, so we don't support it", that's a perfectly good answer and I'll drop it — the crash was the real problem and that's fixed. I'm asking because the failure mode flipped from loud to silent (an under-reported cost rather than an aborted turn), and because the same shape just landed in packages/core via #43251, so whichever way you want it, it probably wants to be the same in both.

Happy to send a patch if you'd like the string path kept.