Clean up YDB adapter internals and regression fixtures

[KirillKurdyukov]

Summary

Targeted cleanup approved after the independent maintainability review of #270.
This remains on Trino 483, based on main 94955bb; the separate 484 draft
#271 is unchanged.

• Remove unreachable non-Decimal JDBC fallback branches, the orphan varchar
  helper, unused type aliases and unreachable Decimal AVG registration.
• Inline the one-caller RewriteUtils; use existing unary pattern guarantees.
• Reuse UPDATE primary-key metadata for RETURNING without changing key-update
  rejection, row counts or transaction ownership.
• Reuse production Guice wiring in fixtures, overriding only the fixture
  client; remove the empty metadata subclass while preserving the custom
  factory's false policy and caching lifecycle.
• Replace SQL-substring PK detection in fixtures. Preserve the synthetic DDL
  case for primary key, assert YDB rejects this forbidden name, and exercise
  real CREATE/INSERT/SELECT with the valid quoted name primary_key.
• Strengthen parameter-order fixtures with distinct operands, clarify the
  unsupported timestamp-literal contract and exercise both real bootstrap
  variants and Decimal metadata mapping.
• Fix example launcher paths and documented CLI service invocation.

16 files changed: 167 insertions, 257 deletions. Two unnecessary classes removed.
Native type guards, JOIN semantics, MERGE resource/failure coverage, capabilities
and existing inherited suites remain intact. No tests are skipped to go green.

Verification

JDK: Temurin 25.0.2.

JAVA_HOME=$(/usr/libexec/java_home -v 25) mvn -B -ntp \
  -f ydb-trino-adapter/pom.xml \
  -Dtest=TestYdbExpressionRewrites,TestYdbColumnMappings,TestYdbJoinMappings,TestYdbMergeSink,TestYdbWriteMetadata,TestYdbPlugin \
  clean verify

Final local result: 27 passed, 0 failures/errors/skips, fresh package built.
All integration sources compile. The actual public connector factory starts
and shuts down for production and fixture module variants.

Regression evidence

• Temporarily swapped NULLIF/strpos operand parameter groups and wrongly
  admitted an in-range timestamp predicate: exactly the three intended tests
  failed by assertion. Restored production code: all six then-current
  expression tests passed. No mutation remains.
• The new fixture DDL case failed before its fix with a missing hidden PK;
  the same case passes after removing substring detection.
• Existing Decimal writer assertions now exercise the actual client mapping,
  including TYPE_NAME precedence over generic JDBC precision/scale.

Example preparation

Executed the real start.sh and both real Maven commands. Only Docker
state-changing commands were intercepted; the interceptor recorded their
working directory/arguments and invoked the real Compose config validator.
Old script failed because docker/ had no Compose file. Fixed script staged
79 JARs under examples/trino/plugin, validated the actual config and
confirmed service trino. bash -n, Compose validation and git diff --check
pass; the executable bit is preserved.

This proves packaging/config preparation, not running example containers.
Docker/Colima was not restarted or repaired. All temporary shims, staged JARs,
mutations and the unused worker worktree were removed.

Java/Bash language servers are unavailable; compiler and shell/Compose
validation were used. The pre-existing obsolete Compose version warning
remains. Local real-YDB execution requires the unavailable Colima daemon;
the ordinary full canonical CI is the final runtime gate.

Final full CI result

Run 36858049165
passed for exact head 49ebcb4d4363b71b986c96d22e73586e7f8ed838.
Actions tested merge checkout dd3931c against unchanged main 94955bb.
Temurin 25.0.4; ordinary workflow command in ydb-trino-adapter:
mvn $MAVEN_ARGS clean test.

368 total, 283 passed, 85 skipped, 0 failures/errors; BUILD SUCCESS.
The 85 exclusions are unchanged from the baseline, not added by this cleanup.

Group	Total	Passed	Skipped
Inherited connector contract plus custom cases	274	193	81
Smoke contract	35	31	4
Native JOIN/type/boundary cases	26	26	0
Production CREATE/INSERT/DML	6	6	0
Unit/API/bootstrap regressions	27	27	0

Both final live identifier outcomes passed: quoted primary_key
CREATE/INSERT/SELECT and exact rejection of the forbidden primary key.

Initial failure and correction

Run 36853742332
on 9570b6169afd0b549584ce190e1c947593c15f8f: 368 total, 282 passed,
85 skipped, 1 error. The sole error is the newly added positive live test
for the column primary key. YDB rejects spaces in column names even when
quoted: Invalid name for user column 'primary key'. Its generated DDL
correctly contains the hidden PK, so generator correctness and backend name
acceptance must be tested separately.

The documented naming rules
allow Latin letters, digits, - and _, at most 255 characters, excluding the
system prefix __ydb_. The task owner approved correcting the mistaken new
test expectation. Commit 49ebcb4d4363b71b986c96d22e73586e7f8ed838 preserves
the synthetic DDL regression, asserts the exact real rejection, and uses
quoted primary_key for CREATE/INSERT/SELECT, with cleanup on all outcomes.
All 27 local regressions, clean packaging and the final full CI pass.
No skip or name-remapping feature was introduced.

Review and delivery

The complete integrated diff was self-reviewed against the approved scope.
No new dependency, generic framework, type-support expansion, retry or
statement-atomic MERGE claim is introduced. An attempted isolated wiring
worker failed before execution; the primary agent implemented and verified
that work instead.

Final-head CI and recorded self-review are complete. Ready for review;
not merged and no automatic merge requested.