[refactor](fe) Merge fe-connector-api into fe-connector-spi

[morningman]

What problem does this PR solve?

Problem Summary:

The connector plugin contract was split across two modules whose names were
inverted relative to common usage: fe-connector-api held the types a plugin
IMPLEMENTS (Connector, ConnectorMetadata and its Ops sub-interfaces, the
scan / write / procedure providers, handles, pushdown and ddl value types) —
which is what "SPI" normally names — while fe-connector-spi held mostly the
engine services a plugin CONSUMES (ConnectorContext, ConnectorStorageContext,
ConnectorConf), which is what "API" normally names. Both modules' package-info
already documented the inversion and asked readers to "read the content, not the
name". The fe-connector-metastore-api / -metastore-spi pair right next to
them follows the opposite (standard) convention, so one directory carried two
contradictory naming rules.

Swapping the two names does not fix it. A strict split by "who implements" is
circular: ConnectorProvider.create takes a ConnectorContext (spi -> api),
while ConnectorContext.createSiblingConnector returns a Connector
(api -> spi). That cycle is exactly why Trino keeps its whole plugin surface in
a single trino-spi module, and this framework is modelled on Trino.

This commit does the same: fe-connector-api is merged into
fe-connector-spi, and org.apache.doris.connector.api becomes
org.apache.doris.connector.spi. The merge is dependency-neutral — every module
that depended on fe-connector-api (hive, hudi, maxcompute, paimon, trino,
fe-core) already depended on fe-connector-spi as well.

Two consequences worth calling out:

• The connector plugin API version is bumped 2.0 -> 3.0. Every type on the
  contract changed its fully-qualified name, so a plugin built against 2.0 must
  be refused by ApiVersionGate at load time rather than fail later with
  NoClassDefFoundError. Both recorded baselines
  (connector-metadata-methods.txt, connector-plugin-surface.txt) and the
  version pinned in ConnectorPluginSurfaceTest are updated in this commit.
  ConnectorPluginManagerTest used to hardcode the compatible major as a
  literal; it now derives both the stale and the current major from the
  kernel's declared version, because what that test asserts is the gate
  (stale refused, current admitted), not which number is current.
  ConnectorPluginSurfaceTest remains the one deliberate speed bump.
• ChildFirstClassLoader.DEFAULT_PARENT_FIRST_PACKAGES listed only
  org.apache.doris.connector.api., so ConnectorProvider and
  ConnectorContext were not parent-first. After the merge the single
  org.apache.doris.connector.spi. entry covers the whole contract.

The merged package-info.java keeps the existing Rule 1-7 design rules; Rule 4
now explains why both directions live in one module instead of disclaiming the
inverted names. README.md and AGENTS.md are updated accordingly.

Release note

None

Check List (For Author)

• Test: Unit Test
  • Full FE build passes, including fe-core test compilation.
  • Every unit test under org.apache.doris.connector passes: 314 test
    classes / 1421 cases, 0 failures, 0 errors, across the 58-module
    reactor (fe-core included).
  • Checkstyle passes across the whole fe reactor.
  • Both architecture gates pass: check-fe-connector-imports.sh (plus its
    self-test) and check-fe-core-metadata-funnel.sh.
  • No regression test run: this is a pure rename/merge with no behavior
    change.
• Behavior changed: No
• Does this need documentation: No

🤖 Generated with Claude Code

[hello-stephen]

Thank you for your contribution to Apache Doris.
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Please clearly describe your PR:

1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
2. Which behaviors were modified. What was the previous behavior, what is it now, why was it modified, and what possible impacts might there be.
3. What features were added. Why was this function added?
4. Which code was refactored and why was this part of the code refactored?
5. Which functions were optimized and what is the difference before and after the optimization?

[morningman]

run buildall

[morningman]

run buildall

[morningman]

run buildall

[hello-stephen]

FE UT Coverage Report

Increment line coverage 100% (0/0) 🎉
Increment coverage report
Complete coverage report

[github-actions]

PR approved by at least one committer and no changes requested.

[github-actions]

PR approved by anyone and no changes requested.

[morningman]

skip buildall