feat!: remove the deprecated mcp-bridge plugin

[shreemaan-abhishek]

Description

The mcp-bridge plugin was marked as deprecated in #13640, with a note that it would be removed in a future release. This PR removes it.

Changes:

• Delete apisix/plugins/mcp-bridge.lua and its apisix/plugins/mcp/ modules (server, SSE transport, shared-dict broker), which no other plugin uses.
• Drop it from the default plugin list, conf/config.yaml.example, the Makefile install target, and t/admin/plugins.t.
• Delete its docs page and sidebar entry, and the README mention.
• Delete its tests (t/plugin/mcp-bridge.t, t/plugin/mcp-bridge.spec.ts, t/cli/test_mcp_bridge.sh) and the CI step that built the filesystem MCP server they needed (ci/prepare_filesystem_mcp.sh).
• The mcp-session shared dict is now declared only when openapi-to-mcp is enabled, which still uses it.

Users who still list mcp-bridge in their plugins config need to remove it. openapi-to-mcp is the MCP plugin going forward.

Which issue(s) this PR fixes:

Fixes #

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)

[membphis]

LGTM