## Summary

Adds error-path support for flexible HTTP outcalls in the consensus
payload. Previously, only the success path
(`FlexibleCanisterHttpResponses`) was handled. This PR introduces a new
`flexible_errors` field on `CanisterHttpPayload` to carry three error
types through consensus:

- **`Timeout`**: The request exceeded `CANISTER_HTTP_TIMEOUT_INTERVAL`.
Flexible timeouts are now reported via `flexible_errors` (instead of the
regular `timeouts` vec) so they can later be encoded as
`FlexibleHttpRequestResult::Err` with a `global_error`, enabling
programmatic detection. For now, flexible timeouts do not carry
information, however, the type is designed so that it could carry more
information in the future (e.g., it could contain all rejects/error
responses).

- **`TooManyRequestErrors`**: More HTTP adapter nodes returned reject
responses than the slack allows (`committee.len() - min_responses`),
meaning `min_responses` OK responses can never be reached. Carries the
reject response+proof pairs for validation.

- **`ResponsesTooLarge`**: Even the smallest `min_responses` OK
responses (by `content_size`) exceed `MAX_CANISTER_HTTP_PAYLOAD_SIZE`
when summed. Carries the metadata shares as proof. The validation logic
still has a bug, which will be fixed in CON-1709.

This addresses CON-1691, CON-1692, and CON-1693.

### Key changes

- **`find_flexible_result` replaces `find_flexible_responses`**
(`utils.rs`): A single scan now sorts shares by `content_size` ascending
(favoring smaller responses), collects OK and reject responses, and
returns one of `OkResponses`, `Error(TooManyRequestErrors |
ResponsesTooLarge)`, or `Pending`.

- **Payload building**: The main loop integrates `find_flexible_result`
and routes flexible timeouts to `flexible_errors` instead of `timeouts`.

- **Payload validation**: A new validation block for `flexible_errors`
checks:
  - Timeout: request is actually expired.
- TooManyRequestErrors: entries are rejects, from unique committee
members with valid signatures, and the reject count exceeds the allowed
slack.
- ResponsesTooLarge: shares are from unique committee members with valid
signatures, and the smallest `min_responses` entry sizes truly exceed
the payload limit.

- **Validation refactoring** (`utils.rs`): Common per-entry and
per-share validation logic is extracted into
`validate_flexible_response_with_proof` and `validate_response_share`,
eliminating duplication between OK-response and error-response
validation.

- **`CountBytes` precision fix** (`canister_http.rs`):
`CanisterHttpResponseMetadata::count_bytes()` now uses precise
field-by-field calculation instead of `size_of::<Self>()`, which was
incorrect for heap-allocated fields like `CryptoHash` and
`ReplicaVersion`.

### Not included

`into_messages` conversion of `flexible_errors` into Candid-encoded
`FlexibleHttpRequestResult::Err` will be done in a follow-up.