feat: usage report transaction IDs and the usage history

[fuzcap]

What does this PR change?

The SDK side of two Kaiten features: idempotent usage reports (transactionId) and the usage history. Server side: kaitencloud/kaiten#16.

• transactionId. UsageReportInput.TransactionID makes a report idempotent: Kaiten applies it at most once per key, instance and entitlement within its idempotency window (35 days by default). A malformed key fails locally with ErrInvalidTransactionID; an empty one is left out of the body.
• Instances.ReportEntitlementUsage returns a UsageReportResult: the usage, plus Replayed (Idempotent-Replayed) and MetadataDropped (Kaiten-Metadata-Dropped). ReportEntitlementUsageMetric is now that call without the two flags.
• Retries, only with a key. A keyed report is retried on transport errors and on 500, 502, 503 and 504: three attempts in all, every one under the same key, with exponential backoff that honours the context. A report without a key is still sent once, because a failed request may have been counted.
• Conflicts. A 409 wraps ErrTransactionIDReused or ErrThresholdExceeded according to its problem code, alongside the *Error. ReportUsage no longer reports a reused key as a threshold.
• Older servers. A Kaiten without transactionId refuses the field (the body is closed). The client then resends the report without it and stops sending keys and retrying for its lifetime, logging that once. WithLogger sets the logger; slog.Default() otherwise.
• Usage history. Instances.ListUsageReports walks a pair's history (From, To, TransactionID, a Limit ceiling). Instances.ExportUsageReports and ExportOrganizationUsageReports stream CSV or NDJSON: the body is returned unread, with the attachment's file name.
• internal/gen is regenerated from the current Core OpenAPI document, and the report wrapper passes the new 503 problem through.

Testing

task test           # ok (-race -shuffle=on)
task lint           # 0 issues
task deadcode       # nothing unreachable
task release:check  # compatible additions only; suggested v0.1.0

• Body: transactionId sent only when set.
• Result flags read from the headers.
• Retries: 503, then a transport error, then 200 gives three attempts under one key. Without a key: one attempt. Always 502: gives up after three. No retry on 409, 422, 404 or 403. A cancelled context stops waiting.
• Conflict codes map to the right sentinel, through both ReportEntitlementUsage and ReportUsage.
• Malformed keys are refused with no request.
• An older server's 422 on body.transactionId gives a resend without the key, later reports keyless and unretried, and one log line. A newer server's InvalidTransactionId is not mistaken for it.
• History pages followed by afterSeq, the Limit ceiling, query parameters.
• Exports: the body streams through with its file name and content type; problems arrive as *Error with their Code; organization filters and a malformed ID option.

task vuln reports standard library advisories of the local Go toolchain (go1.26.3, fixed in 1.26.6), not of this module.

Compatibility

Additive only (gorelease: no incompatible change). Keys need a Kaiten release with transactionId; against an older one the SDK falls back as described above.