chore: run tests by default in Maven builds

[GOODBOY008]

Motivation

<maven.test.skip>true</maven.test.skip> has been a POM default ever since the EasyExcel import (f7d8ceb). This inverts standard Maven behavior and made -DskipTests a no-op (maven.test.skip takes precedence in both the compiler and Surefire):

Command	Before	After
mvn clean install	tests neither compiled nor run	tests compiled and run
mvn clean install -DskipTests	no-op — tests still skipped	standard: compile tests, skip execution
mvn clean install -Dmaven.test.skip=false	the only way to run tests	no longer needed

This deviates from ASF practice:

• The Apache parent POM defines no test-skip defaults; projects inherit Maven's default lifecycle where the test phase runs.
• Sibling projects (POI, Commons, Maven core) all run tests by default; the only same-default upstream is alibaba/easyexcel.
• Surefire's Skipping Tests page positions maven.test.skip as a last-resort CLI escape hatch ("If you absolutely must"), not a POM default.
• Our own docs were misleading: the contributor guide suggested -DskipTests "to skip unit tests" and the release vote templates asked voters to build with -DskipTests, although the ASF Release Policy requires voters to "compile as provided, and test the result".

Why now (history: #390 → #392)

#390 flipped this default once (2025-06) and #392 reverted it the same day because CI took 2–5 hours. Both causes are gone:

1. With tests enabled at the POM level, the install CI step back then re-ran the whole suite a second time (its core/** scope limit did not exclude the large-file tests — large/ lives under core/). This PR keeps that step on -DskipTests, so the suite runs exactly once per JDK job, same as today.
2. The historical test weight has shrunk on its own: measured from 2026-10-15 CI logs, the full suite (1051 tests then, 1085 now) takes ~2.5 min per JDK job; LargeDataTest including the 74 MB large07.xlsx now runs in 89 s. Multi-hour CI durations these days are runner queueing, not test execution (one 236-min run broke down as 232 min queued / 2–3 min executing).

Changes

• pom.xml: drop the maven.test.skip default.
• .github/workflows/ci.yml: the test step no longer passes -Dmaven.test.skip=false (tests run by default); the follow-up full-reactor install step now passes -DskipTests so the suite does not run twice.
• .github/workflows/nightly.yml, .github/workflows/fuzz-tests.yml: drop the now-redundant flag.
• .github/copilot-instructions.md: update the build/test commands.
• Website docs (en + zh-cn): mvn clean install is documented as the primary build command; the release deploy command and vote templates no longer skip tests.

Verification

• ./mvnw clean package -pl fesod-common,fesod-shaded,fesod-sheet (no flags): 1085 tests run, 0 failures, BUILD SUCCESS in 1:16 (JDK 17) — tests now run by default.
• ./mvnw clean package -pl fesod-common -DskipTests: "Tests are skipped." — standard flag semantics restored.
• ./mvnw spotless:check: clean.
• Net CI work is unchanged: the PR test matrix executes the same suite it already ran via the explicit flag.

Note for reviewers: #1227 moves .github/copilot-instructions.md to the root as AGENTS.md; that PR will need a trivial rebase after this one merges.

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.