feat: add MaxAge and TTLRemaining to CandidateResponse

[mvanhorn]

Summary

Adds MaxAge() and TTLRemaining() methods to fsthttp.CandidateResponse and deprecates the ambiguous TTL() method, bringing the Go SDK in line with the Rust SDK's cache-TTL surface.

Why this matters (#250)

CandidateResponse.TTL() reports maxAge - age (the remaining freshness), but its name does not make clear whether it returns the full freshness lifetime of the cached object or the freshness left right now. The Rust SDK already deprecated its TTL accessor and split the concept into MaxAge ("how long the object stays fresh") and TTLRemaining ("how much freshness is left right now"). This change mirrors that surface so callers can distinguish the two.

Changes

• MaxAge() (uint32, error) — returns the full suggested freshness lifetime (opts.maxAge, or the set TTL when SetTTL/CacheOptions.TTL is in effect).
• TTLRemaining() (uint32, error) — returns the remaining freshness, maxAge - age, and saturates to 0 for already-stale candidates instead of wrapping the unsigned subtraction.
• TTL() is retained as a thin wrapper over TTLRemaining() and carries a // Deprecated: doc comment so existing callers keep working.
• All three read from the existing getSuggestedCacheWriteOptions() result, so there is no ABI change.

Testing

• go test ./fsthttp/... passes.
• go vet ./... and gofmt -l are clean on the changed files.
• integration_tests/httpcache builds for GOOS=wasip1 GOARCH=wasm.
• testAfterSendCandidateResponsePropertiesUncached now asserts MaxAge() returns the full suggested lifetime (3600) and TTLRemaining() returns maxAge - age, alongside the existing TTL() backward-compatibility assertion.

Fixes #250

[dgryski]

Minor suggested tweak, but otherwise LGTM.

Thanks!

[dgryski]

I think I'd prefer this if block to look like

		opts, err := candidateResponse.getSuggestedCacheWriteOptions()
		if err != nil {
			return 0, err
		}
		maxAge, age = opts.maxAge, opts.age
        if candidateResponse.useTTL {
           maxAge = candidateResponse.overrideTTL
        }

[mvanhorn]

Done in 00b3a4c. fsthttp tests pass locally.

[dgryski]

LGTM!

[mvanhorn]

Appreciate you landing this, @dgryski. Good to get MaxAge and TTLRemaining in and the ambiguous TTL deprecated.