zookeeper_proxy: migrate config validation to exception free

[davidvo-lyft]

The zookeeper_proxy network filter was one of the few remaining network filters whose config factory still threw EnvoyException instead of returning absl::StatusOr, which kept its whole directory on the tools/code_format/config.yaml exception allowlist and outside the no-throw check. This change switches ZooKeeperConfigFactory from Common::FactoryBase to Common::ExceptionFreeFactoryBase so createFilterFactoryFromProtoTyped returns absl::StatusOr<Network::FilterFactoryCb>, converts the two config-time throws (the duplicated-opcode check and the unknown-opcode lookup) to absl::InvalidArgumentError with byte-identical message text, and removes the zookeeper_proxy entry from the exception allowlist so check_format now enforces no-throw for the entire directory. The ZooKeeper filter's data plane was already made exception free in #31485; these two config-time throws were all that remained, so this finishes zookeeper_proxy specifically. It continues the exception-free migration tracked in #27412, following the same pattern as recently merged network-filter (#46111) and HTTP-filter (#46042) work.

Commit Message: zookeeper_proxy: migrate config validation to exception free

Additional Description: parseLatencyThresholdOverrides becomes a public static helper returning absl::StatusOr<LatencyThresholdOverrideMap>; the factory parses before constructing ZooKeeperFilterConfig, whose constructor now takes the parsed map and can no longer fail.

Risk Level: Low. Config-load-time only refactor with no data plane change; error messages and failure semantics are unchanged (EnvoyException becomes absl::InvalidArgumentError with identical text, surfaced through the same config rejection path).

Testing:

• //test/extensions/filters/network/zookeeper_proxy:config_test and :filter_test pass in the CI clang docker image. The duplicated-opcode case was converted from EXPECT_THROW_WITH_REGEX to a status matcher with the identical message substring; a new UnknownOpcodeInLatencyThresholdOverrides test calls the now-public static helper directly with an out-of-range opcode and asserts InvalidArgumentError with the identical text, covering the error branch so per-directory coverage does not regress. filter_test's existing tests run unchanged through the updated initialize() helper.
• Countable metric: throw sites in the directory go from 2 to 0 (grep -rE 'throw|THROW'), and the allowlist entry is removed. Mutation proof: with the allowlist entry removed, injecting a throw into filter.cc makes check_format fail with "Don't introduce throws into exception-free files"; at the parent commit the same injected throw passes, confirming the directory is now enforced.

Docs Changes: N/A

Release Notes: N/A (internal refactor, no behavior change; matches the no-changelog precedent of #46111 and #46042)

Platform Specific Features: N/A

Part of #27412

Generative AI disclosure: Developed with AI assistance (Claude Code); I reviewed and understand every line and take full ownership of the submission.

[gemini-code-assist]

Caution

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[davidvo-lyft]

/retest