Apply Java naming conventions and behavior-preserving idiom cleanup

[marevol]

Summary

Behavior-preserving refactor that brings the java-saml sources in line with
standard Java coding conventions and cleans up inconsistent / non-idiomatic
code. No external interface changes — every public/protected class,
method, field, constant, and enum keeps its name, signature, and value, so
downstream consumers compile and link unchanged. The full test suite passes
with the same counts as before (core 415, toolkit 92, 0 failures).

Scope was limited to naming fixes, safe idiom modernization, and dead-code
removal (no functional/behavioral changes, no default-value or security
changes).

Naming

Renamed only local variables, private fields, and private methods that
violated Java naming conventions; public accessor names were left exactly
as-is for compatibility:

• The trigger case: the leading-uppercase local AssertionDataNodes →
  assertionDataNodes in SamlResponse.
• snake_case → camelCase (cert_begin/end_cert → certBegin/certEnd).
• acronym casing on private fields (nameID, spX509cert,
  wantXMLValidation, uniqueIDPrefix, idpx509cert, …) — public getters
  such as getWantXMLValidation / getUniqueIDPrefix / getSPcert are
  unchanged; only the private fields they read were renamed.
• typos (errorAcumulator, stausCode, childs, decompresser,
  idpDescryptorXPath, idvalue, …).

Idioms (behavior-preserving)

• size() != 0 → isEmpty(); x != null && !x.isEmpty() →
  StringUtils.isNotEmpty(x).
• Hoisted per-call HashSet construction into static Set.of(...) fields.
• if (…) return true; return false; → return <expr>;; if/else
  assign-then-return → ternary.
• Dropped redundant boxing / .toString() / parentheses / generic
  type-witnesses.
• Explicit "UTF-8" / Charset.forName("UTF-8") → StandardCharsets.UTF_8
  (byte-identical output).
• Parameterized SLF4J logging ("… " + v → "… {}", v) with identical
  rendered text and identical throwable handling.
• Canonical modifier order public static final (was public final static)
  on the SettingsBuilder property-key constants — names, types, and values
  unchanged.

Dead code

• Removed an unused LOGGER field and its imports in IdPMetadataParser.
• Deleted a commented-out method block and an unused parameter in
  SettingsBuilder.
• Removed dead commented-out debug lines in SamlResponse.

Samples and tests

• Fixed broken package imports in the sample JSPs — they referenced a
  non-existent package (a leftover rename artifact) and could not have
  compiled at runtime.
• Corrected stale @see javadoc package paths in AuthTest (comment-only).

Compatibility and safety notes

• One null-contract subtlety was handled explicitly: after hoisting the
  algorithm whitelist to an immutable Set.of(...),
  Util.isAlgorithmWhitelisted(null) is guarded to keep returning false
  (an immutable set would otherwise throw on contains(null), where the
  previous HashSet returned false).
• The public API surface was independently reviewed to confirm no accessor
  name, constant name/value, return type, or throws clause changed.

Testing

• mvn -B test — core 415, toolkit 92, 0 failures / 0 errors / 0 skipped,
  matching the pre-refactor baseline (no tests added or removed; behavior
  unchanged).