Which issue does this PR close?

• Closes feat: expose ConfigOptions.set/get as generic SessionContextBuilder.setOption / SessionContext.getOption #48 .

Rationale for this change

SessionContextBuilder (introduced in #28) currently exposes typed setters for the six most-common session knobs: batchSize, targetPartitions, collectStatistics, informationSchema, memoryLimit, and tempDirectory. The Rust DataFusion configuration surface backing these is roughly 200 keys split across seven sections under SessionConfig (datafusion.catalog.*, datafusion.execution.*, datafusion.optimizer.*, datafusion.sql_parser.*, datafusion.explain.*, datafusion.format.*, plus user extensions). The Java builder reaches none of those except the six it names explicitly, and there is no Java surface to read any config value back at all.

DataFusion already exposes a string-keyed setter for these (ConfigOptions::set(key, value)). This PR mirrors that surface in Java rather than adding ~200 named setters one at a time.

What changes are included in this PR?

• Two additions to the existing SessionOptions proto: a new repeated ConfigOption options = 7; field plus the ConfigOption { key, value } message. No existing field changes; wire format stays backward and forward compatible. repeated is used instead of map<string, string> because protobuf maps decode into HashMap on the Rust side, whose iteration order is randomized — that would silently break overlapping-key cases like datafusion.optimizer.enable_dynamic_filter_pushdown (which has side effects on the per-operator enable_*_dynamic_filter_pushdown flags).
• Two new methods on SessionContextBuilder:
  • setOption(String key, String value) — single entry.
  • setOptions(Map<String, String> entries) — bulk apply.
• One new method on SessionContext:
  • getOption(String key) → String — read the current value of any datafusion.* config key. Returns the value as a string, or null if the key is recognised but has no value set and no default. Throws RuntimeException on unknown keys (mirrors setOption's strictness for fast feedback on typos), and IllegalStateException when called on a closed context.
• Native side: free-form options are applied via config.options_mut().set(k, v)? before the SessionContext is constructed, so the typed batchSize/targetPartitions/etc setters can be overridden by an explicit setOption for the same key. getOptionNative walks ctx.copied_config().options().entries().

The Rust calls use the ? form (rather than SessionConfig::set_str(...).unwrap()) so unknown keys or unparseable values surface as a RuntimeException on the Java side carrying DataFusion's error message, instead of panicking the JVM.

What's intentionally out of scope: datafusion.runtime.* keys

datafusion.runtime.* keys (memory limit, temp directory, cache sizes, list-files-cache TTL) live on a separate RuntimeEnv config object. Round-tripping them through getOption/setOption has several upstream-shaped correctness pitfalls that don't apply to the session-config subtree (lazy default tempdir creation that materializes a path the user never set, per-session datafusion-Xxxxxx spill suffixes, OS-specific path separators, integer K/M/G truncation that loses fractional capacities, sub-1KB values formatted as bare bytes, the unlimited sentinel, and disk-manager state being clobbered when multiple runtime keys are routed through SET). Doing them right needs a per-context side-cache of the user's verbatim values.

This PR rejects datafusion.runtime.* in both setOption and getOption with a clear error pointing at the typed memoryLimit / tempDirectory setters. Adding round-trippable runtime support is tracked as a separate follow-up.

Apply order for setOption

Map entries are applied after typed setters by design — otherwise a caller writing both batchSize(8192) and setOption("datafusion.execution.batch_size", "1024") would have their explicit setOption silently dropped. The opposite order would make setOption strictly weaker than the typed setters and give callers no way to override.

Among setOption calls themselves, entries are applied in caller insertion order (first call applied first). The Java side stores entries in a LinkedHashMap, the proto field is repeated (preserves order on the wire), and the Rust side iterates a Vec (preserves order on apply). Caller's last write wins for both same-key duplicates and overlapping side-effect keys.

Why getOption lives on SessionContext, not the builder

The value getOption returns is "what DataFusion actually compiled". That's only knowable post-construction, so the read-side method belongs on the constructed SessionContext. A builder-side getter would only return what's pending in the local map, which is a strictly worse signal.

Are these changes tested?

Yes — 14 new tests in SessionContextBuilderTest, bringing the file from 5 to 19 tests total. All pass.

Are there any user-facing changes?

Yes — purely additive. New public API:

• SessionContextBuilder.setOption(String, String)
• SessionContextBuilder.setOptions(Map<String, String>)
• SessionContextBuilder.setOptions(LinkedHashMap<String, String>)
• SessionContext.getOption(String) → String

The new options field is also exposed on the existing org.apache.datafusion.protobuf.SessionOptions generated class (getOptionsMap() etc.). No API removals, no deprecations, no behavior change for callers who don't call any of the new methods.