refactor: drive datafusion.runtime.* config from a ConfigField schema

[haochunchang]

Which issue does this PR close?

• Closes chore: Explore combining SessionConfig and RuntimeConfig into same framework #23322.

Rationale for this change

The eight datafusion.runtime.* keys were spelled out by hand in six places: the
private RuntimeConfigValues struct and its into_config_entries, the value
derivation in RuntimeEnv::config_entries, a hardcoded default table in
RuntimeEnvBuilder::entries, and a hand-written match arm per key in both
SessionContext::set_runtime_variable and reset_runtime_variable. A comment in
set_runtime_variable asked contributors to remember to update
reset_runtime_variable when adding an option, and the hand-rolled parse arms are
the code that produced the panic fixed in #23316.

#23322 asks whether runtime config can use the same type-checked framework as
SessionConfig/ConfigOptions instead of its own hand-rolled SET/RESET path.
This PR gets there for the parsing/dispatch framework: RuntimeOptions is built
with the same config_namespace! / config_field! macros that back
ConfigOptions, so SET/RESET now dispatch through ConfigField and adding a
runtime option is a one-line struct field instead of edits in six places.

RuntimeOptions deliberately stays a separate struct rather than fields on
ConfigOptions itself: datafusion-execution depends on datafusion-common,
never the reverse, so ConfigOptions (which lives in datafusion-common) cannot
hold it. It is also not a ConfigExtension, because extension prefixes may never
be datafusion. Fully merging the two option sets is the "independent project"
the issue itself calls out as a separate, harder problem (runtime config is
shared across SessionConfigs and can change mid-execution); this PR only
unifies the framework they're built on.

What changes are included in this PR?

• Add datafusion_execution::runtime_options::RuntimeOptions, the single source
  of the eight datafusion.runtime.* keys, their descriptions, defaults and
  parsing.
• SessionContext::set_runtime_variable / reset_runtime_variable now build a
  RuntimeOptions::default() and dispatch through ConfigField::set, instead of
  a hand-written match per key. parse_capacity_limit keeps its signature,
  doctest and behavior, delegating to the shared parser.
• RuntimeEnvBuilder::entries reads its defaults from the same constants the
  builder uses instead of hardcoded strings such as "100G", removing a silent
  drift risk between the builder's actual defaults and the table shown to users.
• RuntimeEnv::config_entries is now a one-line delegate to
  RuntimeOptions::env_entries, which owns the datafusion.runtime namespace
  prefix and the unlimited-pool special case; from_runtime_env is private.
• Three per-key listings remain (the schema, the from_runtime_env read of the
  live resource objects, and the apply_key write onto RuntimeEnvBuilder) and
  are now all in one file, runtime_options.rs. The last two can't merge into
  the schema: the builder setters have per-field signatures, and reported values
  must come from the live resource objects rather than from whatever was last
  requested.

Error text changes as a side effect: ConfigField::set isn't given the key it's
setting, so a leaf parser can't name the key mid-message the way the old
hand-written arms did. Errors now carry the key as a when setting '<key>'
suffix, which also makes the wording consistent — the previous messages used
for '<key>' in some arms and when setting '<key>' in others.

Public API is unchanged: RuntimeEnv and RuntimeEnvBuilder keep every public
field and setter.

What is the testing strategy for this PR?

• datafusion/execution/src/runtime_options.rs unit tests (parsing, defaults,
  SET/RESET round-trips for every key) — cargo test -p datafusion-execution --lib runtime_options.
• Existing datafusion/core/tests/sql/runtime_config.rs integration tests,
  updated for the new when setting '<key>' error wording — cargo test -p datafusion --test core_integration runtime_config.
• Existing datafusion/sqllogictest/test_files/set_variable.slt cases, updated
  for the same error wording.
• SessionContext::parse_capacity_limit doctest.
• Confirmed docs/source/user-guide/configs.md is unchanged: ran
  ./dev/update_config_docs.sh and diffed — the generated runtime table is
  byte-identical, so it isn't touched by this PR.
• cargo fmt --all -- --check and cargo clippy --all-targets --all-features
  (datafusion-execution and datafusion) are clean.

Are there any user-facing changes?

Only error-message wording for invalid SET/RESET datafusion.runtime.* values
(now uses a consistent when setting '<key>' suffix — see above). No behavior,
default, or public API change.

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